Not entirely. There are third party TB controllers by now, the problem is they're heavily guarded by NDAs.
Also, I heard that Thunderbolt 4 is the same as Thunderbolt 3, except all the optional features from 3 are required in 4.
This interview seems relevant: https://www.youtube.com/watch?v=lnSMbgndFHo&t=2m3s&feature=y...
Not entirely. There are third party TB controllers by now, the problem is they're heavily guarded by NDAs.