There's perfectly reasonable reasons to mention IDE, for example some companies have propriety Eclipse (or Emacs) extensions to support their code base or use collaborative work capabilities that are integrated with IDEs.
Environmental setup also has an impact, a company might use project files that automatically setup the environment and set common standards. Sure you can come in and rewrite all of those things in your favourite IDE which you then have to manually sync when things change, but that's a lot of overhead.
There's also some people who will only work at companies that use their favourite editor (particularly common in Java with IntelliJ) and will ignore job ads which mention Eclipse.
It can also act as signalling (i.e "we're willing to pay for you to have the best-of-breed development tools") much like jobs ads which specify developer monitor sizes or hardware (Macbooks, SSDs, etc).
I agree with a lot of what you're saying actually, it's why this rule emits a notice rather than an error or warning. Considering that some of the rules are a little polarising, it would probably make sense to allow people to ignore certain rules. Thanks for the feedback
If you do pair programming, you may find it desirable to establish a common development environment, so pairs don't experience friction between one person and another's development environment. In that case, you might want to hire people who know that tooling, or, more likely, you might want to filter out people who have a passionate attachment to some other tooling.
Environmental setup also has an impact, a company might use project files that automatically setup the environment and set common standards. Sure you can come in and rewrite all of those things in your favourite IDE which you then have to manually sync when things change, but that's a lot of overhead.
There's also some people who will only work at companies that use their favourite editor (particularly common in Java with IntelliJ) and will ignore job ads which mention Eclipse.
It can also act as signalling (i.e "we're willing to pay for you to have the best-of-breed development tools") much like jobs ads which specify developer monitor sizes or hardware (Macbooks, SSDs, etc).