No, it wasn't, and hasn't been since the 90s (most of the gvim code was written by others back in the day, to give an example of a large body of code not written by Bram). There have been many people contributing, and some of them for years.
Yes, Bram was the BFDL and the only one committing patches, but don't confuse that with "one man show".
There were of courses patches by other people, but most big features were driven by him. Do you have some example of a big feature more recent than 20 years old gvim?
Most of the multibyte from the last few years for example.
Bram didn't really record these things very well until quite recently (and even then, rather inconsistently) by just listing the author in the commit message rather than the git author field, so there's no easy way to check.
