Does this work for custom in-house extensions?

The checker covers third-party extensions only, in-house custom modules need a per-module audit. Same verdict scheme applies: read the module’s frontend code, check whether it uses Knockout / RequireJS, count the number of phtml templates that reference Magento’s frontend JS APIs, estimate the Alpine port effort. As a quick rule of thumb, a custom module with no frontend code is automatically Compatible; one with 2 or more phtml templates and AJAX is almost always Partial. Send the module list via the audit form for a written verdict per module.