Totally fair point, and I admit I did "Um..." and "Ah..." over that specific line a bit - it could maybe be "fair" at least.
I suppose I partly went with that since while I can't really "read" Japanese - as in, I don't really have a proper idea what the style of language the author is using is like, unless they're going mad with slang, ye olde words, or something like that - I still felt pretty confident there were a lot of things about the writing here that pointed towards this not being totally srs business. If you haven't looked at the raws, it doesn't do pop culture references or suddenly start wacky slapstick or anything, don't worry, the tone stays fairly downbeat and the action is still gritty/grim. But there's absolutely stuff in 2.1 and 2.2, at the very least, where I'd be astonished if the mangaka was like "No! You're not supposed to be laughing!"
I might be getting it "wrong" in some way, for sure... but as far as I can tell, nobody here is "de gozaimasu"-ing it up. I haven't played Sekiro but it actually made me think of Blade of the Immortal? Which I really liked before it started going nuts with every other female character getting the #sexualviolence tag, and it used a fair few anachronisms - the old Dark Horse localisation made it clear "Yeah, they speak like this in the original Japanese, too" - and didn't mind cracking a smile now and then.
I'll stop - that was probably a lot more text than you really needed, heh. Just wanted to show my working a bit, I guess, even if I'm not a proper translator.