I’ve never heard this about Julius Caesar, but maybe about Tiberius… as well as one of the big name Philosophers - Plato maybe - where all we have is what his student wrote.
Regardless if I have those names right, there are plenty of people and events in history that are only mentioned once, or there are completely conflicting reports, yet historians don’t doubt these people existed or these events happened.
All those rules seem to get changed when it comes to Jesus Christ. Suddenly the numerous writings from eye witnesses are thrown out as ‘biased’ and even the writings of ‘secular’ historians of the day are nit-picked and thrown out.
If we apply merely the same historical standards that we do to the accounts of say Hannibal’s invasion of Italy or the Battle of Thermopylae or of Mohammad what we find is that something truly world changing happened in a back-water part of the Roman Empire around 33AD due to a lowly carpenter.
Like no other event in history, the events surrounding Jesus changed the world. Problem is most historians - like most people - don’t like their live challenged. If they were to study the life of Jesus like they would study other history, their lives would be challenged.