According to Fightful Select, a reason that several MLW stars like Brian Pillman Jr. were able to work for AEW while still under contract with MLW was due to there being no clause for AEW when the original contracts were issued. While most companies prohibit their talent from working for their competition there was no language written for AEW specifically since it did not yet exist.
The report does note that some new contracts were written an offered to MLW talent, but no word on whether or not anyone who hasn’t already committed to staying have taken it. Meanwhile Pillman Jr, who regularly appears on AEW programming, has publicly stated that he plans to honor his remaining MLW dates despite being denied a release request earlier this year.