My first reaction would be to say this is native, however, that would be based on my previous experience with Office Addins for Word and Excel which was quite some time ago and I also don’t remember browser engines being involved. With this in mind I went to check the MSDN docs on Office Addins, in particular, for Outlook and the situation seems to be a bit trickier.
According to this:
the scripts are executed in a browser engine in a sandbox so this still seems to hint at native.
However, then the docs on authentication hint at the use of implicit flow which is not really aimed for native applications.
… your add-in prompts the user to sign-in to the service either by using the displayDialogAsync method to initialize the OAuth2 flow, or by using the office-js-helpers library to the OAuth2 Implicit flow.
In addition, I’m a bit unsure, but it seems these addins would also be able to run in Outlook Web and not just Outlook desktop so if that’s so it would muddy the waters even further. In conclusion, if these would run only in a “browser” engine sandbox in the end-user machine then it should be native; something akin to Electron. However, the specifics for Outlook addins may mean this needs to be handled differently. Given you already worked with these addins you may want to consider updating the question with additional context information, for example, example on how the addins can be executed (desktop only, embedded in the outlook web application, etc).