Anybody has an online resource where I can understand lambdas better? thanx

Yes, including me, now. I just had to repeat the attempt until the random test let me pass.
I went with Seq.sortBy, which should behave the same as Array.sortBy - but obviously doesn't.
Would you be so kind as to include a short passage about standard collection functions mayhaps behaving differently to each other in the F# version used by Codewars? That would give a hint at what the problem may be.

sadly, this is not tail recursive, as the last operation is the multiplication, not the recursion. Potentially, this might cause a stack overflow, even though you should hit the integer overflow first.

Nice because you did not need an inner function.
Not so nice because it isn't tail recursive and may cause a stack overflow.