This is true, however I think an App Store rule that to ship a browser engine, you have to be a browser, defined as having a browser that is maintained on MacOS, Linux, and/or Windows and which can be made the default browser on those platforms. Or even simpler, it has to present web browsing to the user as the primary function and not secondary to accessing content/shopping/gaming.

Seems either approach would rule out your Slack, Amazon app, etc. from shipping their own outdated 900MB Chromiums but allow Chrome, Firefox, K-Meleon, whatever.