Last I had heard, Tesla has opened their superchargers up to all? brands for use. People just need to get an appropriate adapter (if it exists).I think this may have more to do with the charging network than the cars themselves. I was road tripping in the US during the thanksgiving long weekend (busiest travel weekend of the year) and only once had to wait to use a supercharger. The CCS chargers I saw were all always in use with many cars waiting. It must absolutely suck to road trip in a non-Tesla EV on busy weekends. I wouldn't buy a non-Tesla EV now for that reason. But ask me again in a few years after the rest of the charging infrastructure catches up and the superchargers open up to non-Teslas and it'll be a different story.
Plenty of videos online of Tesla owners raging at other brands taking up a SC spot.