And that doesn't conflict with the possibility that some combination of explicit mental training, meditation, psychedelics or other chemical and biological agents can permanently expand those boundaries.

If intelligence comes from neural connections it might be enough to spam compounds that create a lot of new ones and then do exercise to reinforce them while the unused connection naturally decay. A kind of simulated annealing for the brain. If you don't get smarter on the first try you might be able to repeat the process until it works. You only need to get it once.

I think a lot of us are stuck with the idea that something that is fixed in nature will always be fixed even after we try to change it.