Hi Seth,
Yes, 'tools' would normally be included in this guidance.
An example of a tool that might bend this rule to some degree is FEMU. We don't directly develop FEMU, but we do have a lot of influence there.
An example of a tool that was shot down in the past is a (externally developed) json formatter. In that case we didn't develop it, didn't have much influence over the development of the tool, and whether it was truly required (vs a nice-to-have) was arguable.
IMO, proposals for including tools should not be shot down out-of-hand, but the default answer will be: don't include the tool (i.e. the proposal would need to demonstrate required value that can't reasonably be addressed in another way).
All that said, we have situations like the "private gn sdk" which adds google internal only tools/code on top of the gn sdk (which is on top of the IDK). I.e. depending on the audience there are workable approaches other than having something directly in the IDK.
Hope that helps!