I interpreted it as you don't heat the pot while the frog is still in the pond. You only apply heat slowly once the front is already in the pot.

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There was an experiment once where it was determined that a frog with it's brain removed wouldn't jump out of slowly heated water but would reflexively jump if placed into already hot water, leading to a myth that a frog won't leave boiling water if heated gradually enough.

Idioms around frog boiling generally means to make changes slowly and gradually enough that there is minimal reaction from affected parties.

I guess the frog has to get into the pot first is their point? I agree that the metaphor is not doing a very good job at conveying whatever was meant though.

Google slow boiling frog