I've seen a lot of takes on the GPL but this is a new one to me. As another comment points out, you only have to make changes available to others if you make the software itself available to others; either by distributing a binary or by using it to provide a web service.

But this is, fundamentally, the point of "Free" software: someone who uses a piece of software should be entitled to change that software as they desire. This obviously implies that if you make your own modified software available to others, they too must be able to make their own changes.

The AGPLv3 only requires you to make your modified source code available to people that you choose to give a copy of the binary to or allow to use on your computer over the network. You never have to make anything available to the original owner if you don't let them use your modified version.

> That sounds like proprietary copyright wherein third parties are allowed to view, but not modify, the source code, to me.

... Except that they're literally allowed to modify the source code?