My pet peeve is people calling a 2560 x 1440 monitor "2K". It already has a real name QHD that describes the resolution why try and force "2k" onto it when it doesn't even make sense. Do you know what actually is a 2K monitor... 1920 x 1080. The exact thing you were saying was bad. 1920 is a of a lot closer to 2K compared to 2560. The funny thing is even Wikipedia has a list of
2K Resolutions and surprise surprise 2560 x 1440 is not on the list, but 1920 x 1080 is on that list.
If you absolutely had to use the "K" nomenclature and you were to round 2560 to the nearest 1000 it would be a 3K monitor. Round to the nearest 100 it would be a 2.6K monitor.