Under employment laws you can quit basically at any time with given notice and you can apply to any job no matter who you are or what you did before. The non compete clauses are always part of the employment contract. Usually, what’s in the contract is binding, but: there’s things that might be voided upon examination. Here things like consideration and unconscionability come into play. I assume this clause would be ruled unconscionable against employment laws, therefore the clause is basically removed from contracts after the fact and precedent allows for it to be voided upon future use.
employment laws > contract law. That’s all it boils down to I assume, just what weighs more.
A lot of European countries allow only very limited non compete clauses or none at all. Moving in that direction is not really without precedent, so there’s your legal argument.
Also obligatory IANAL, if you think I’m wrong and you got sources, please correct me. I wanna learn what I don’t know.