If you make a MIT project "Foo", not protected by a CLA and I contribute to it, then if your company uses Foo in a proprietary product they still have to give the attribution for my code as required by the MIT license, as my code will still remain MIT even if embedded in a larger non-MIT project.

OTOH if I had signed a CLA, then your company would be within its right to entirely strip any attribution to give whatever license they see fit to the code I wrote.

Yeah, they'll have to attribute the code to you - the MIT license will always apply - but they'll only need to attribute you - they can make as many modifications as they like.

The proprietary+forked code would become a mix of closed and open source license, but just never get released so you’d never be able to get ahold of it.