Non-competes might be difficult to enforce legally, but getting caught violating one would be a good way to kill any opportunities to work in entertainment in the future.
I don't think we have definitive proof if Cover include non-competes, but I would be very surprised if they didn't, at least for a period of time. From the company's perspective, taking on a new talent involves making an investment (upfront costs plus the cost of marketing them) with the intention to later receive a return on that investment (the revenue share). They would not want a contracted talent to also maintain an active vtuber account outside of the company that grows through word-of-mouth thanks to the exposure Cover have paid for, as said talent could then graduate the company persona at the first opportunity, depriving the company of the ROI.
I personally dislike non-competes. I have witnessed contractors continue working in poor conditions because the mandated "break" after they leave would be damaging to them financially, and I've also enjoyed seeing contractors I know give a confident "well, fuck you then" and quitting in response to harsher non-competes being added to their contracts when a renewal was due. But I also understand why someone running a business would include them for a limited period of time, to ensure new recruits aren't planning to promote themselves at the company's expense, then bolt before they've earnt that money back.