You're right of course. The Publisher's Preface is a big deal. Paragraphs 2 and 3 show how they thought Russell was the Laodicean Messenger and faithful steward "beyond question of a doubt". The scans of the 1918 edition are at https://archive.org/details/TheFinishedMystery, in case you're interested.
From the Publisher's Preface:
Through St. John the Lord Jesus revealed the fact that
the Church would be developed during seven distinct
periods, or epochs; and that for each of these epochs
He would have a special angel, or messenger, to
serve the other members of the Body. It follows,
then, that the messenger to the last, or Laodicean,
epoch would declare the Presence of the Lord and the
time of the Harvest of the Gospel Age. The great
Master laid special emphasis on the importance of the
messenger to the seventh, or Laodicean, period of the
Church, saying that such an one would be "a faithful
and wise servant, whom his Lord would make ruler
over all His Household, to give them meat in due
season."
Those consecrated Christians who have read and
fully appreciated the Truth as contained in the
preceding six volumes of STUDIES IN THE
SCRIPTURES readily see and agree that Charles
Taze Russell was the messenger of the Church of
Laodicea. In the mind of every one who believes the
Bible the evidence set forth in this volume will
establish that fact beyond the question of a doubt.