I'm a little confused about this part. Does the Online Safety Act create personal liabilities for site operators (EDIT: to clarify: would a corporation not be sufficient protection)? Or are they referring to harassment they'd receive from disgruntled users?
Also, this is the first I've heard of Microcosm. It looks like some nice forum software and one I maybe would've considered for future projects. Shame to see it go.
This seems very plausible to me, given what they and other moderators have said about the lengths some people will go to online when they feel antagonised.
The UK has lots of regulatory bodies and they all work in broadly the same way. Provided you do the bare minimum to comply with the rules as defined in plain English by the regulator, you won't either be fined or personally liable. It's only companies that either repeatedly or maliciously fail to put basic measures in place that end up being prosecuted.
If someone starts maliciously uploading CSAM and reporting you, provided you can demonstrate you're taking whatever measures are recommended by Ofcom for the risk level of your business (e.g. deleting reported threads and reporting to police), you'll be absolutely fine. If anything, the regulators will likely prove to be quite toothless.