one would just say it's using symbolic language there.
That is frequently given as a fall-back position in discussions on apocalyptic books, but it is methodologically specious. Not everything in an apocalyptic book is "symbolic," indeed because it is intended to reveal things unseen, the revelation is aimed at explaining and showing things. This is the case throughout apocalyptic literature; the Book of Parables in 1 Enoch contains a detailed description of the geography of the world and the Book of Luminaries similarly purports to explain the dynamic workings of the heavenly luminaries in heaven. More to the point, what is God symbolic of in Revelation? Or what is Jesus Christ a symbol of? Similarly, what are the "souls of those slain" supported to be symbolic of, if they are not the "souls of those slain"? The reference to these souls in ch. 6 anticipates the later reference to the "great crowd" in ch. 7 (which are innumerable, whereas the souls must wait until the full number is reached) and the "souls of those beheaded" in ch. 20 who are resurrected in the first resurrection. So if these souls are only symbolic of something else, then the resurrection itself is symbolic and ch. 20 thus does not really have anything to with a literal resurrection. This is the slippery slope that results from an ad hoc attribution of apocalyptic statements as symbolism; whether something is intended to be figurative is sometimes established through the context and sometimes it is ambiguous. But there is no ambiguity here because Revelation discusses the slaughter of martyrs at many points in the book and the "tribulation" is certainly one that has lethal effects, as its description in ch. 13 entails. The gathering of the elect in heaven is a prominent theme, as is their later resurrection. What we have in ch. 6 is a reference to the intermediate state between death and resurrection, and one would have to be hard-pressed to say what that scheme is supposed to be symbolic of, if not literally referring to martyred Christians and their waiting for their vengeance. Moreover, another apocalyptic book, 4 Ezra, was written around the same time, and it similarly refers to the souls of dead waiting for the end, in rather similar language to what is in ch. 6 of Revelation.