Who says they didn't believe it was possible? They just didn't think it was possible for you, i.e. that you did it honestly.

We all know cheating is a problem in online games, so it wasn't an unreasonable belief. It's certainly possible to play chess at Magnus Carlsen's level, but if I play against someone random online and they play at that level, then of course my default assumption is going to be that they're cheating, not that they're Magnus Carlsen.