Yawn alert: While, as with all of us, my main motivation for participation in this forum is interest in the natural sciences, particularly from a biological perspective, the tech nerd part of me finds this issue interesting. So if anyone beginning to read this feels an oncoming sense of slumber, I will take no offense if you read no further. 
It seems likely to me that the most effective way for the system to conclude that you have read all posts in a thread that has at least 100 posts is for it to keep a tally of what URLs you have visited within the thread. After you have visited all of the URLs in the thread, the process of awarding the badge is initiated and completed. When there are some short posts at the end of a thread, it is possible to read the last one without its URL appearing the address bar. However, there is a remedy for this.
Note that when you have scrolled past the first post in a thread, and it has scrolled off the top of the window, the URL in the address bar ends with a number, a slash, and then a final number. The first of those two numbers is the sequential number of the thread, and the second one is the sequential number assigned to the post within that thread. If there are several short posts in your window at the same time, the URL displayed seems to be the one corresponding to the post in the uppermost portion of the window. Therefore, to display the URL of the final post in a thread, you need to scroll up sufficiently to vacate all the preceding posts from the window.
So, @cs16-levi, note that as of this time, the final post in the “Beware: AI Images on inat” thread that you referenced above belongs to you. Its URL is the following:
https://forum.inaturalist.org/t/beware-ai-images-on-inat/44346/121
For the system to determine that you have read all the posts in that thread, you must scroll down far enough for that URL to appear in your browser’s address bar. You may even need to scroll past that post.
The tech nerd part of me, and possibly those of a few others here, would be interested in learning of how this works out for you. 
Happy reading!