1: it's possible his brother figured it out through trial & error or just using the blank template
2: brothers talk to each other a lot, we know that stickman talked about f35b so he probably talked about this stream as well, especially if it was one he was most active on
3: brothers, as you may know, come from the same family and thus will learn the same way, so it's likely they just reason the same way because that's how they were taught.
there's plenty of room for doubt, and none of this is evidence, just questions and suspicions. in short, all of this "evidence" is in your head