It depends upon what you mean by opinionated. There is software that expects users to behave in certain ways and there is software where the developer is quite firm about what they will and will not implement. I would say that calibre and kitty fit into the latter category. These applications offer a lot of features and are very configurable, yet they also don't try to be everything to everyone.
While I don't know why Goyal takes the approach he does, I would imagine that a lot of demands are placed on him simply because his software is so powerful (and, in the case of calibre, pretty much the only program in its domain that goes beyond serving basic needs).
How is the dev a jerk when they don't want to invest time into patching their app for the quirks of another app? Especially when kitty seems to have tmux-abilities out-of-the-box. I mean, you can like their solution or not, but calling them a jerk for not supporting your way?