They mentioned 2 months as you couldn't apply to be listed until you had traded at least 2 months from what someone said before.
Also txbit.io daily volume is bigger than quite a few exchanges that are all ready listed, Although I don't think the biggest traded coins on txbit.io are listed on CMC so that could be an issue.
Listing requirements of CMC changed over time, so it is truly not the same case for all exchanges at different period.
Sometimes, CMC give easy listing requirements.
In contrast, sometimes CMC adjust their requirements and it turn to harder to get their acceptance to list coins/ tokens/ exchanges on CMC.