This is, at best, speculation.
Quickly following on the heels of the Fall was a time when men stopped caring about the Lord. To assume that people remained faithful to a command to be vegetarian is naive.
Your viewpoint is as speculative as mine, as the fruit and vegetation before the flood grew greatly and was watered from below the ground, all various kinds of edible fruits and nuts and plants were in abundance and nobody would have needed to eat meat.
However, whether some did or didn’t, what is fact is that permission for peole to eat animals was not given until after the Flood, and the fear of man was not placed in animals until after the Flood.