It’s certainly possible that sayings of other people were later attributed to him, but to really make this case you’d need to have quotations that were attributed to multiple sources, including him, if you see what I mean. Absent that, it could be true, but there’s no particular reason to believe it.

There are enough specific biographical details about Jesus of Nazareth to make it likely that there’s a specific, real central figure. For example, the fact that he was from Nazareth was a problem for his early followers (it didn’t match the Messianic prophecies), which is why they invented the odd story of the census, so that they could claim he’d been born in Bethlehem. That seems unlikely to have happened if there hadn’t been a real, central historical figure.

Also, none of the early non-Christian sources claim he wasn’t real, which they surely would have done if there was any doubt on the matter.