(I'll also note that it's somewhat odd to hear this response from someone whose entire mission in life is essentially to go meta on all of humanity's problems...)

That's not the kind of meta I mean. The dangerous form of meta is when you spend several years preparing to do X, supposedly becoming better at doing X, but not actually doing X, and then try to do X. E.g. college. Trying to improve at doing X while doing X is much, much wiser. I would similarly advise Effective Altruists who are not literally broke to be donating $10 every three months to something while they are trying to increase their incomes and invest in human capital; furthermore, they should not donate to the same thing two seasons in a row, so that they are also practicing the skill of repeatedly assessing which charity is most important.

"Meta" for these purposes is any daily activity which is unlike the daily activity you intend to do 'later'.

Tight feedback loops are good, but not always available. This is a separate consideration from doing meta while doing object.

The activity of understanding someone else's proofs may be unlike the activity of producing your own new math from scratch; this would be the problem.