There are some songs, like How Soon is Now, which I think is totally him and how he felt at the time, and I could be completely wrong about that too.
I not very emotional about him and the Smiths and I am not heartbroken and all that dramatic stuff, but I recognize the emotion in the songs, if that makes sense.
He is just a great writer, imo and is really able to write about emotions and human experiences in ways people can really connect, but I completely agree with you that they are not his entire life story, but there are some-
Hairdresser on Fire
Mr Shankly
Bigmouth
are just a few
But then you have songs like National Front Disco that are about a situation he wants to say something about, he just writes about a variety of things, like any good songwriter should
I think people need to believe all the songs are autobiographical because they are somewhat fanatical and want to feel a stronger connection to him