British and Continental Literalism predates Millerite Adventism by centuries. If we define Adventism as any belief in the near return of Christ, then yes, Russellism is Advenism. However, Adventists and Literalists in the Russell era strongly rejected each others theology. Literalists refused the designation Adventist. This continues though with different designators. Fundamentalists believe in the ear return of Christ. We do not define them as Adventists, and in fact, they are not. They stand in the Literalist tradition.
When in 1881 a small congregation in Kingston, New York, was described as Adventist one of their members responded to the local newspaper:
Lest the grave charge of numerical insignificance be inadequate to the complete extinction of a ‘half dozen’ religious worshipers, they must be brought into the inquisition again to be placed upon the rack and be thrust through with the deadly charge of being ‘akin to the second adventists’! We were not aware of any kinship existing between us and the second adventists, without it could be established upon the isolated truth of the personal second advent of Jesus to this earth. But mark you, if that isolated truth can establish a kinship between us it will also prove and establish a kinship between Rev. Dr. Lorimer [then a prominent Baptist clergyman] and the second adventists, and, by your curious and extraordinary method of gauging a man’s standing, it would place him, as well as the ‘six in the small upper room in the American Block,’ under the ban and the fetters of social and religious ostracism. For his sermon on ‘the future of Jesus’ is a scholarly, elaborate and eloquent vindication of the doctrine of the personal, visible and premillennial second advent of Jesus to this earth. [Original spelling and punctuation retained.] - The Halfbreed Church, The Buffalo, New York, Evening News, July 12, 1882.
Russell and millennialist clergy were careful to distinguish themselves from Millerite Adventism. The reverse was true. Adventists were careful to distinguish themselves from Age to Come millennarianism. Storrs left Adventism in 1844 but continued to teach the near return of Christ. L. C. Gunn, a prominent Millerite, wrote of Storrs' meetings in Philadelphia that: "These were the published views of Geo. Storrs. ... In these views they differed entirely from Mr. Miller and the great body of Advent believers in this country, but agreeing with the Literalists of England (Millennarians)." - Quoted in I. C. Wellcome: History of the Second Advent Message and Mission, 1874, page 382. For similar matter making the same distinction see for instance, Amos Sanford: Controversy, The Restitution, June 23, 1875.
This distinction is made repeatedly into the 20th Century. But more importantly, it was strongly made by Russell and Adventists when discussing the other's views. For instance in the July 18, 1877, issue the Advent Christian Times warned against their work:
"One N. H. Barbour, called Dr. Barbour, with his confreres, J. H. Paton and C. T. Russell, is traveling around the country, going everywhere that they can find Adventists, and preaching that Jesus has come secretly, and will soon be revealed and mingling in their lectures a lot of “Age-to-come” trash, all to subvert their hearers. They are not endorsed by Adventists, “Age-to-come” folks, or anybody else, yet having some money and a few sympathizers they will probably run awhile. They have been to Ohio and Indiana and are working westward. We are credibly informed that one of them boasted in Union Mills, Ind., a few days since, that they would break up every Advent church in the land. We guess not. Their whole work is proselytizing. The Lord never sent them on their mission. Give them no place, and go not near them or countenance them."
Schulz and de Vienne discuss the distinction at length, and they probe the origins of Age to Come Literalism. You should read the Separate Identity series. If we wish to be honest historians, we will acknowledge the definitions of each movement as they saw themselves. Defining any belief in the near return of Christ as Adventism is inexact at best. Do we call Campellites Adventist because Campbell strongly advocated the doctrine? Do we call Evangelicals Adventists because they see his return as near? Hardly. So do we define Russell as an Adventist despite adopting Literialism, a far older theology than Millerite Adventism? We should not.
I highly recommend R. M. de Vienne's introduction to volume 2 of Separate Identity. It's too long to post here, and I do not have permission to do so. But it is the clearest of all advocates for a distinction between Russellism and Adventism based on origin of doctrines, self-definitions of the various parties involved, and a detailed examination of those things within Russellite theology that are supposed to be Adventist.