~6 months ago, they were completely proprietary. It's being opened up now, but it doesn't change the history. Tesla home chargers would not work on anything else. The cars came with an adapter to allow them to use J1772 chargers. The adapters to use a Tesla level 1/2 charger on a J1772 car are still hard to find, and mostly from sketchy sources.
Tesla was also forced to adopt (or at least become compatible with) CCS2 in Europe. It's not unreasonable to think that it could happen in the US. Releasing NACS puts them ahead of the curve rather than behind.