If pocketing opponents last ball illegally , opponent can choose any pockets for the 8 ball.
ILLEGALLY. If you hit your ball first and pocket theirs, that's legal by any standard definition of billiard rules.
If you are stripes and shoot the 10 ball in, then the cue ball hits their 6 in, it's still your turn. Alternately, if you hit your 10 ball into the 6 and it touches a rail, that's a legal shot. Your turn is over, but you did NOT take a foul. To cut it to it's simplest form, a legal shot
A: requires you to shoot the cue ball into one of your balls, then sink one of your called balls, or
B: Shoot the CB into any of your balls then ANY ball (including CB) hits a rail.
That poster is disproving your stance.