> The fact that the canon wasn't fixed doesn't mean the apocryphal books were part of it

That's not the point. The point is that there wasn't a single canon, different sects of Jews had different canons. All these canons were overlapping – all Jews accepted the Torah, there was widespread (but not universal) acceptance of the Prophets; the third part of the Jewish canon, the Writings (Ketuvim) saw the most disagreement. And we have evidence that the canon of the Qumran community did include books now considered "apocryphal"–see https://www.jstor.org/stable/24663170

Did some Greek-speaking Jews include "apocryphal" works in their canon, just as the Qumran community did? Well, we have evidence that the Qumran community included Greek-speaking Jews – most of the Dead Sea Scrolls were in Hebrew or Aramaic, but some Greek texts were recovered as well.

We know that many Greek-speaking early Christians accepted some of the pre-Christian Greek "apocryphal"/"deuterocanonical" works as canonical. Why did they do that? Well, a very plausible hypothesis is that some Hellenistic Jews already accepted them as canonical, and Hellenistic Christianity inherited that acceptance from (segments of) Hellenistic Judaism. We don't have direct proof of that, but as a historical hypothesis it is very plausible, especially in light of the evidence from Qumran that some Jews (even Greek-speaking ones) did accept (other) "apocryphal" works in their canon.

> And that's besides the fact that even if there was some so-called "debate" about it, if the canon didn't actually change as a result of those debates (e.g. Song of Songs and Eccl. are part of the canon), then how can you say it wasn't fixed?

The canon did change. In the middle of the second century CE, the Pharisees still had multiple canons – a narrow canon which excluded Song of Songs and Ecclesiastes, a broad canon which included them, and also a couple of intermediate canons which included one but not the other. The outcome of this debate was that the broad canon won out over the narrow and intermediate ones, and supporters of those other canons died out.

And, if we consider other Jewish groups such as the Sadducees or the Qumran community, we find even more Jewish canons than that. However, the 1st and 2nd centuries CE saw a great deal of decline in the diversity of Judaism, due to various factors (the trauma of the Jewish-Roman wars, competition from Christianity), and part of that decline in diversity was the reduction of multiple canons to one. But that reduction did not complete until after Christianity had already split off from Judaism, which is why many Christians (both in the early Church, and also today) do not believe that Christians are bound by it.

And why do you dismiss it as 'some so-called "debate"'? It was a real debate, you can read the Mishnah for yourself – https://www.sefaria.org/Mishnah_Yadayim.3.5?lang=bi&with=all... – allow me to quote some of it:

> All the Holy Scriptures defile the hands. The Song of Songs and Kohelet (Ecclesiastes) defile the hands. Rabbi Judah says: the Song of Songs defiles the hands, but there is a dispute about Kohelet. Rabbi Yose says: Kohelet does not defile the hands, but there is a dispute about the Song of Songs. Rabbi Shimon says: [the ruling about] Kohelet is one of the leniencies of Bet Shammai and one of the stringencies of Bet Hillel.

All the Rabbis it quotes are known to have been active in the 2nd century CE. So in the 2nd century CE, there was a real debate among the (successors of the) Pharisees about the canonicity of Song of Songs and Ecclesiastes.

Furthermore, this passage gives evidence that the same debate was active in the 1st century CE as well (and possibly even the 1st century BCE too), through Rabbi Shimon's reference to Bet Shammai and Bet Hillel.