He's grabbing at straws so Jaeui doesn't leave.
Jaeui and Rahman have their own story/book, so... Yeah, Rahman was using Jaeui's reluctance to make weapons as a reason to keep him there with him, and while it offers him his company it still bothers him immensely that Jaeui's reasoning for most things are for Taeui, not himself. That's why he despises Taeui, because it feels like he's the only thing Jaeui sees and that they're destiny bound, while Rahman is trying anything abd everything to tie Jaeui to him in any form, albeit in not very nice ways.
Now the one thing he relied on to keep him with himself is taken away and Jaeui drew weapons, so Rahman's new excuse will be "Oh I didn't know you were THAT good, now I can't let you leave" when really he wasn't gonna willingly let him leave, weapon or not, because he knows once he lets him leave he's probably never going to see him again.
True, but honestly my immediate thought to this was "oh no what a nightmare" sarcastically because this is the least scandalous thing anyone in this manhwa has ever done till now lmao. No one blinked when Ilay existed just as himself but we draw the line at Rahman going back on a promise because he can't stand watching the guy he's been yearning for dor years walk away...
The comments were just getting too funny for me that's all, but I do understand the frustration.

That mf needs a beating from ilay. How dare he scam the twins after he draw him a weapon. Like what? You won't send them off but you'll get the blueprint? Shameless b4st4rd hope you d!e son of a b.