except of "doesn't matter, these are just terms" it would make me a bit sceptical if they know what they are talking about.
in hybrid martial arts its differemt. i know of the korean art tong-il-moo-do, which is described as a mix of karate, judo and taekwondo, that they use japanese terms like dojo, kata etc.
2) It might be a remnant from pre-unification Korean Karate
Case 2 is less likely, and if it is the case, it's likely to be only a few terms for a few specific techniques, and not words like sensei and dojo, which have had a clear and accepted Korean equivalent even since before the war.
It would have to be evaluated on a case-by-case basis. Gnarlie points out two viable possibilities. I'll add a third possibility in that it's personal preference due to the totality of training. As an example, I teach Kong Soo Do Karate. It's pretty much redundant on purpose. I started off in Okinawan arts and that is my root. Later on I trained Korean (as well as Israeli, Japanese, Chinese etc). In essence, a lot of training methodology went into what I/we teach. I chose a Korean name for the art to honor my longest association with my instructor who is a Korean arts master. But, and this isn't meant in any disrespectful manner, I don't use Korean terminology at all. I prefer Okinawan terminology as it was the first I learned and those arts still have a profound impact on what I now teach. So as with the arts name, I honor all of my training and former instructors in some way, shape or form by what and how we teach.
Secondly, though rare, I have seen Korean masters still use Okinawan terminology and call themselves Sensei. Perhaps this is to honor the roots from which their art came? Perhaps personal preference?
As with anything, names and terminology are secondary to good teaching. If the teaching is sound then that is the primary goal. I've often joked that I could call my art 'Dave's Bait, Tackle and Kung Fu Emporium'. As long as my students find value in what I offer the name doesn't matter.
My instructor used both both Korean and Japanese terminology, for things such as bow, teacher, school, form. For the names on techniques we use English terminology. My instructor taught Tae Kwon Do at a school that was mainly Kenjutsu a Japanese Sword style. Some students came to Tae Kwon Do from Kenjutsu so the terminology has just been somewhat combined ever since. I mean my instructor even calls it Karate (Korean) some times. I think it isn't the biggest deal in the world to call a Sabum , a Sensei, but Sifu would be a little odd. There are plenty of Tae Kwon Do school in my area that use the Korean Karate name and use the Japanese terminology. I was just seeing people's thoughts.
Many times I've seen the terms, Korean Karate, Tae Kwon Do Karate and Tang Soo Do Karate. I think that is a very respectful way to identify the roots of the art while also maintaining individuality. I wouldn't have an issue with any of those if they were offering the type of training I was looking to take.
We can look at things in a Korean school beyond terminology that aren't actually Korean. For example, belts are of Japanese origin. The uniform is of Japanese origin, though the Koreans adopted a slightly different one in some schools. The Dan/Kyu level was of Japanese origin and was adopted over to Korean training. So any terminology that remains really shouldn't be of that much concern. Again, all boils down to the training.