Sadly, even with the nice backstab, He could have had two attempts of sleeping you just about. Backstab is 2 rounds of lag, he can flee;re enter;sleep twice if they are fast enough before your lag dissipates. A safer command would be to just murder target, because you know he's going to try and flee and sleep 99% of the time. Best to start with murder because it doesn't lag as long, and by the time your lag clears, you can enter it again to catch him before his movement lag is countered.

Of course, even with summon, summon is 1 round lag, so if you entered it almost immediately after he summoned but a small chance regardless. Plus he summoned you into a 1 exit room, thus easy to flee;n;sleep target, because he only had to worry about one exit to go too.