The DALLE-2 images had a very clear watermark.
I would not be surprised to find that NightCafe was inserting identifying marks in their works.
The person you cited seems to have a poor understanding of copyright and fair use. He said:
A lot of AI generated images retain the watermark [...] If you sell something with that image with no agreement from the rights holder it is not fair use.
A watermark is simply a device for tracking the source of things. It has little to do with copyright. Fair use refers to the use of small sections of a larger work. The quote I showed is fair use.
To resell a work, one needs to have an agreement with the creator.