i don't think it matters how expert of an opinon one has when considering confidence on whether someone truly existed or not.

being an expert in history wouldn't help you confidently confirm that anything you read wasn't part of a big popular information conspiracy unfortunately.

their examples of Shakespeare, Socrates, etc. are much more strongly suggestive of being true because of a larger sample size of "historical evidence" from people claiming to exist at the same time as those who wrote about them, and the several events popularly known to be directly caused by them, and not some 50 years removed gospels which may very possibly have been hear-say. (told indirect information, then made a claim based on that)

regardless, it pretty much doesn't matter in philosophy whether someone exists or not since the important thing is the idea associated with the person. the issue is that theology is associated with Jesus, and since theism is a confident belief position, it just doesn't make a ton of sense to live and believe by historical evidence alone. i think complimenting historical evidence with empirical science is a lot more reasonable

to me this would be like if someone had a box, and i really wanted to know what was in it, and they told me it was a carrot and sent me off. now i can believe it was a carrot because they were right there and if they were honest then it should be a carrot in the box, but to personally commit myself to that belief, i would have the see inside the box myself.