Fix logic bug in 1632ea43682f

Fix logic bug in 1632ea43682f

I overlooked that one condition was logically inverted.  The fix is a
little bit more involved than simply negating the condition, to make
the code easier to read.

Fix some outdated comments left by the same commit, while at it.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/YMRlmB3/lZw8YBH+@paquier.xyz