It's perfectly OK however if the founders buy their token after the IPO.
How to enforce this? Every developer could promise to do so and use a middle man to buy during IPO without you knowing.
Let's distinguish developers and founders:
Let's call a 'founder' a person who has a share in funds collected in an IPO, and a 'developer' - a waged person who has no share in those funds, who will be payed for working on the project by e.g. founders.
You are right.
Crypto IPOs are opaque. We can't be sure how much of their tokens respective founders bought from theirselfs.
But some of them at least don't openly state that they will buy their own coins at the last moment, when there is enough info for them to calculate how much BTC they need to transfer from their left pocket to their right pocket, to get desirable share of their tokens.