Yeah, these days it's easy to use an online service to submit to the App Store. Buddybuild (buddybuild.com) is really helpful for this. It acts as a CI system, and it can deploy builds to iTunes Connect with one click.

Bitrise can do that too, and has Xamarin support as well - https://www.bitrise.io/

(I'm the CTO there)

I came here to ask about submitting to the App Store. Does this mean that a Mac is, technically, no longer required for the entire process of development -> release of iOS apps?

You'd probably want to performance test your app with a full MacOS compile, but the whole UI implementation, function and experience testing can be done without a Mac. And you need a Mac for deploying to the store.

There is no mention of this process being able to upload an IPA file so I'm assuming we still need to use xCode or Application Loader on a Mac

They do say "the whole process" https://www.buddybuild.com/features/deployment/

Also http://docs.buddybuild.com/docs/download-ipa

Yes. Buddybuild can build IPA files and upload them directly to iTunes Connect.