You might also let him know that Unix is the root from which both Linux and OS X derive from, not just that they are incompatible. As far as knock-offs, people differ on which is the greater, the various versions of Linux (and BSD, some would argue) or OS X instead.
Different people prefer different things, of course.
You are aware that OS X is not just consider a derivation of UNIX, but is in fact a certified, POSIX compliant Trademarked UNIX itself, one of only four so permitted to be Trademarked, and in fact the best-selling UNIX in the world, aren't you? It is not considered a knock-off, such as Linux, which was created in a "clean-room" environment, reverse-engineering every UNIX command to duplicate the results of that command without duplicating any copyrighted code.
Spell check works in mysterious ways. How did "knock-off" in my original post get changed to "Knox" and why? Even more curious because it was correct when I hit 'Post'. Whose spell checker would have "knock-off", which is a euphemism for both "kill" and "burglarize/rob" AND the word "Knox" with a upper case "K" as in "Fort Knox" in its dictionary as potential substitutions? Secondly, where is this spell checking computer which would possibly be loading my posting and altering the reply? I just tried "knock-off" and variations on my computer and not once did it do a substitution of "Knox" for any possible misspelling I may have made. Weird.
It makes me think more and more there's an intercept server between FR's users and FR which gets our postings first, a man-in-the-middle, listening in, and copying (and censoring) everything before forwarding it to JR's real servers. I know that, in the past, when I've posted the pings, which have had upwards of 500 names on the "to:" list, I'd strangely and frequently and frequently get the ping double posted, as though there were a delay between hitting post and the actual posting, then another would be also sent when I know I had not hit Post twice. I chalked it up to hiccoughs in the network and would request the admins delete one.