A poster on Mumsnet saying 'have you seen the thread on Kiwi Farms' means nothing beyond 'I saw something interesting/alarming about this person on Kiwi Farms',
Exactly.
It doesn't necessarily mean you subscribe to anything on the site. Or maybe you do. As it has been pointed out, there will be people registered on mumsnet who are less than savoury.
Also, how do people warning other people not to look at the site, or else, even know what's on the site?! 🙄