And do they have the number of engineers with the required skills to rewrite half the engine? Especially if the reason why they developed using those tools and engine is they expected not to have to do it themselves in the first place?

It's not like there's just some "go_slow=true" constant that just needs changing.

It's pretty unlikely. A source code license is negotiated with the sales team directly and costs at least USD100k last I heard (the price is not publicly disclosed). They're also reluctant to give source code licenses at all.