The comment says: Every time you click on a link your external IP addresses is exposed, is this a vulnerability? Being online without a VPN / proxy is inherent consent to have your external IP & other required items to be shared with services / middlemen.

The fact that a user's IP is exposed when they click on a link is only relevant to the original post if a user would do this automatically and without realizing. The original post alleges that they can send someone a message on Signal and have the user automatically and somewhat unknowingly load a resource from a server. Sure, the author doesn't claim they have much control over the resource or the server, but they do show how you can check which server the user accessed and how that leaks information about the location of the user to a certain extent.