These “millions of humans” were not accountable, as you very well state.
Therefore (with @Jonathan_Burke‘s words) they were “not responsible to judgment”.
And consequently God didn’t destroy them in the Flood.
The Flood destroyed some hundreds of thousands sinners who despised the opportunity God gave them to atone. These were people living in Noah’s region (the antediluvian Sumer’s cities, as I conjecture), who had corrupted their ways and perpetrate plenty of violence (Genesis 6: 11-12).
For the sake of Redemption and as a warning for future sinners God couldn’t help making it clear that He lets the sinners on earth to give them time to atone. This is the meaning of the Flood.
But to avoid destroying millions of humans God wisely and mercifully waited till the End of the Flood to make all creatures with human body living in the rest of the world responsible for their deeds. God’s mercy is the very reason why He created Humanity in two steps: the first referred to in Genesis 1:27 and the second in Genesis 9:6.
@Kathryn_Applegate has recently very well argued that “Adam was a Real Person in History” and Adam’s Sin a real sin that “became our sin” (see article).
Noah and his Flood is not less real History than Adam and his Sin.
Undoubtedly the record of the flood in the Bible is meant to convey a meaning. It is the meaning Jesus Christ and his Apostle Peter stress in the context of their teaching about the End Times: We sinners are on earth because God wants us to reach His Glory and so gives us time to atone. We should remind this “meaning” every time we see the Rainbow in the raining clouds.