That's the speculation on gut length in pandas based on statistical methods but panda teeth are already well adapted to eat bamboo so selection has been working on them for some time or at least there is no opposing selection at work in panda teeth preventing them from changing. Strangly the large canines are used to cut the bamboo which might be what created their niche in the first place, but their teeth are otherwise very different from other bear teeth.

Its been a while since I dove into this but from what I remember the speculation with gut length has to do with metabolic tradeoff. If pandas don't get that tradeoff with the food they eat then they'll probably keep their current gut length. Or they might make a different tradeoff (slower peristalsis, more gut surface area)

Google tells me that pandas started eating bamboo 6-8 million years ago and stopped eating meat 3 million years ago.

That's not exactly recent.

For reference, the first Homo appeared 2.8 million years ago and the first Homo Sapiens 300 000 years ago.

The last common ancestor of humans and chimpanzees lived 5-10 million years ago.

So if evolution can evolve humans in that time frame, you'd expect that it could also adapt an omnivour to a herbivour.