I doubt whether an exemption is granted to anybody to allow use of an MEL. I can't find any reference to US
operators, but have a look at the extensive information on MELs for foreign operators of
US registered aircraft:
https://www.ecfr.gov/current/title-1...section-129.14
Clearly, the approval for and use of an MEL is embedded in the regs and presumably not an "exemption".
I also severely doubt that there are any 121 operators in the US that don't have an MEL that allows 3 days grace for TCAS.
Something fishy going on.