It's definitely not small but it's a subset of the entire development community. That's not wrong – maybe it allows iterating faster without supporting other languages, for example — but it can also reduce the number of people interested in working on something. Just as a simple example, a Python user can benefit to an improvement in Apache httpd made by a Java, C, PHP, etc. shop but that happens a lot less frequently with Tomcat.
Kind of, but in practice they probably aren't going to use those unless they're already committed to the Java world because they're slower, less compatible with the extensions they're probably using, and fewer people use them so you're more likely to find bugs or gaps in documentation. Again, I'm not saying that Java is terrible or that nobody uses it but simply that the community is smaller than the entire tech world — especially after Oracle's sales pressure — and that means that any given project has a greater chance of not having enough support unless a major company backs it.
The point being that Java-specific things like WARs, Tomcat, etc. were almost exclusively used by Java shops whereas more general tools such as httpd are used across multiple communities. A C++ shop can host their app behind httpd/nginx/etc. the same as a Swift shop even if they don't otherwise use the same tools.
