After some research, it seems like input values are still strings, even if their type is boolean: https://github.blog/changelog/2021-11-10-github-actions-input-types-for-manual-workflows/. Also https://github.com/orgs/community/discussions/29796. We have to account for that in our workflow file.

[I'm still not sure why if: ${{ input.enable_tmate }} would have a problem. I thought GitHub action ifs coerced that type of value.]