I don't think Tae-Yoon even remember he had a night with Eun-Hoo. He probably know what knotting in theory but not on practice and it was his first rut and he's basically 'drunk'. Not to mention he doesn't have any experience in relationship prior and even post meeting Eun-Hoo to begin with.
Eun-Hoo most likely met with his maternal uncle at his father's funeral but they are not close in the first place and the debts are cleared by selling the house so there's no complication afterwards. At least there's someone who seemingly helped Eun-Hoo raising the baby and most likely when he was pregnant. That's why Tae-Eun is comfortable around his 'grandparents'.
It's like calling Ojisan (mister) to older guys but it's also meant uncle.
The '-jisan' also could be used for various calling purpose. ╮(￣▽￣)╭
Allow me to explain a few things.
First, in Ch3, when Eunhoo said Taeyoon didn't even know about the knot, he was referring to when they spent THAT night together. Taeyoon didn't realize he knotted Eunhoo because he was in a RUT. THAT'S WHY HE CAN'T REMEMBER. THAT'S WHY THE POSSIBILITY OF HIM BEING TAE-EUN'S FATHER HASN'T CROSSED HIS MIND. Eunhoo didn't mean that Taeyoon doesn't know about "knotting" itself. Need I remind you that Taeyoon was a college freshman when that happened. He's old enough to understand what knotting is. He might be dense, but he's not that stupid. You also forget his parents pampered him, hence his childish mindset.
Second, Tae-eun referring to the uncle and uncle's wife as grandparents. I saw a comment somewhere saying that Eunhoo's mom and the uncle are siblings. Let me remind you again that Eunhoo's parents divorced when he was still in middle school, and he's been living only with his abusive father since then til the day he died. Now, when his parents are separated, it's clear Eunhoo didn't keep in touch with his mom. That explains why he was struggling alone with the debts and whatnot when his father died. THAT'S WHY THE UNCLE WASN'T ABLE TO HELP HIM THEN. Going back to Tae-eun, as I said, Eunhoo's mom is not really in the picture, which explains why Tae-eun calls them his grandma and grandpa.
Lastly, not every older man in this story is being called uncle. Lol, Tae-eun refers to Taeyoon as "mister" at the beginning of the story up until he received a toy from him. Taeyoon being acquainted with Eunhoo made Tae-eun start calling him "uncle" too. The other reason why he's called uncle because the Korean term "ajji" which is what Tae-eun calls him in the raws, doesn't have any equivalent word in English.
I hope I explained this well. I don't know why some of you could have misunderstood these things. The translation and the way it was presented were pretty clear to me.