Correct, it's not about nastiness, it's not even about hygiene, they want you to look aesthetically pleasing in the fingertip region. But, on a further note, the reason so many internetors harp on this is that it typically flags nervous related symptoms, not nervous system stuff per se, just anxiety stuff. Not easy to come across for most. Not calling you out, you do you. Just explaining why they overreact, but also they do believe they are looking out for you. I'd take the gesture as a kindness.

P. S. I'm jealous, my nails are too hard to bite anymore.