There would have been a multitude of ways to actually improve their security:
an Oauth API, API tokens, token/PIN exchange on the printer's display and/or a QR-code, an actual LAN-only mode, manual certificate exchange, ... I'll stop here.
All of these solutions would be absolutely feasible, actually secure, and would not lock you out from using a third-party slicer. Their solution isn't about security at all, it's only about controlling their ecosystem.