The issue is, another company has forked Zig, made 0, or negative enhancements, and the Zig foundation is making sure to distance itself from the fork incase anyone gets burned by Zen. You don't want the first impression of your new language to be tarnished by some commercial company who dumped everything in marketing and 0 in development.
The GPL wouldn't help in this case at all. It's very clear that the Zig Foundation doesn't care about the changes, it cares about it's reputation. On the contrary having a high quality commercial fork would probably be a good thing for Zig (see MySQL <-> Percona, Cassandra <-> Datastax, Postgres <-> Citus)
“Since then, “No. 2” has resigned from their position at connectFree, but won’t be able to contribute to the Zig project for some time because of a “non-compete” clause present in the contract.“
A malicious actor can pretend that they merged MIT parts with new GPL parts, but I think, it would not take a lot of time, until such merging would become technically hard, and the code can be effectively converted into GPL.
However, since Zen's guy was a contributor, it's probably not possible to get all authors' permission to change the license for the entire Zig codebase.