Any link inside the app that can lead to a purchase page is illegal.
So if they have a button that open their homepage in the app they are at risk. This is insane and stupid monopoly but this is the rule for now.
The Ko-fi solution is to make a donation of at least 6€ at https://ko-fi.com/tolriq without telling anything about license there, then send a private message there or here with the trial ID.