Not exactly true. This is not a definitive statement.
Comparators are available that are not open-collector outputs.

 

Sorry! I didnt know that. But the common ones like 311, 393 and 339 are open-collectors. Mind to give some examples just for knowledge?

Allen

 

While it is true that most commercial analog comparators now come with open-collector or open-drain outputs this was not always the case.

Any op-amp with high open-loop gain such as LM324 or LM3900 can be configured as an analog comparator.

The list of comparators with CMOS, TTL, totem-pole output is way too long to list here.
Here are some examples, TLC1440, TLV3012, AD8469.