Nobody who was paying attention thought he would be a one term president. He said nothing of the sort because that was never his plan. He literally said so at the time:

I don’t have plans on one term

And his campaign emphasized the same thing at the time:

Lots of chatter out there on this so just want to be crystal clear: this is not a conversation our campaign is having and not something VP Biden is thinking about

This is not a conversation we are having among people who actually are running the campaign

And yet people still want to believe a bunch of anonymous “senior advisers” over what Biden actually said. Pure wishcasting.