It may surprise you to know that I also have a good number of Muslim friends and acquaintances - and I have never heard them use the word 'kaffir' - period - a word utilised in SA since the Dutch et English Invasion of the 1600s - so what?
And the reason I called you a pedant is this fine and upstanding contribution by you to my fine and good and also upstanding self;
Now you know the rest of the story. . . .
However,
upon further investigation it would appear we are both correct, to wit. . . .
So, rather than being subjacent, you could have. . . .
Go to gawaher.com. They use the terms, kafir, Kuffar, etc. all the time to refer to non-muslims. I've been called it more than once.