No, I don't like that "gotcha". It instantly broke my suspension of disbelief.
"Otherworld" language? Are only Japanese people getting abducted? Japanese people have been wearing those kinds of clothes for only a few decades, so how many of them have been taken to this world? And how would this guy have met enough of them to identify one? Unless his skill translates any Earth language to Japanese (I don't recall the specifics of the skill) but different languages, and even countries that use the same language, have different idioms, especially across time. Plus Loki didn't start off crazy strong. Sure, more talented than an average adult, but no one from the next town over will have heard of Loki. Unless other isekaiers are given crazy powers or all crash out hard enough to make the national news, I don't know why there would be info floating around about them.
The possibility of this random guild leader learning enough about other isekai'd Japanese-speaking, business-suit wearers to learn their proverbs and recognize habits of "someone who has been transferred to a different world and isn't freaking the fuck out but is trying to assimilate" is too low for that to be regarded as anything other than artifice.
Edit: getting refreshed from the next chapter, it seems like there are significant number of other Terrans kicking around this planet at the same time as Loki, so recognizing him makes more sense.