well on one hand, it is an escalation... but had they previously tried to issue cease and desist orders that were ignored with reasonable doubt onto whether they were successfully issued or not, hiring a regulated agent to deliver the cease and desist in person is probably the more restrained option... they probably could have just filed an injunction. but by the sounds of the story, it sounds like they want to avoid legal injunctions (and otherwise drama) and so they tried to just issue the cease and desist instead.
the article uses words like "strong arm" that may not actually be accurate... the modder may have been surprised and overwhelmed but if TT was choosing between either delivering a cease and desist in person or just filing an injunction, TT actually chose the less strong option. and using a PI is just... a technicality, I suppose, if you have reasonable doubt as to whether or not someone has been receiving a cease and desist... if the modder isn't at his home address (ignoring his mail/phone), and TT doesn't want to have an injunction filed and then the modder subpoena to appear, I suppose hiring a PI to deliver the C&D in person is the alternative. in that role, they'd just errand boys, essentially... for all intents and purpose an intern could do it but you hire a PI because they're a third-party and federally regulated (I assume that's the case in the UK too).
I can't imagine TT asking the C&D agents to do anything other than issue the order... I think a PI could risk his regulatory license for using intimidation, and TT could be found liable and probably negligent (since the PI was acting on TT's behalf) for civil damages...whether or not the modder felt otherwise, I dunno... I mean, maybe he was just scared to see a suit at his door and felt that alone was 'bullying' when the PI would see it as a routine C&D delivery. Not too sure ;p but this could have been a pretty standard issue of a C&D (in a general context) but that seemed overwhelming in the context of a game mod (though it's probably more akin to a hacked private server from TT's POV).