I think it's more about trying to keep your competitors busy defending their moat - so that they don't go on the offense and attack your moat.
Content is the input to the Meta ad engine. You can generate it on-platform, or you can bring it in from elsewhere if it is commoditized. (And commoditizing your complements often looks like breaking down your competitors’ moats.)
I don’t think it’s as cynical as a “salt the earth” strategy though, surely they (plan to) make profit from Threads and LLaMa regardless of what their competitors do? Threads users will drive ad revenue, Llama (theoretically, eventually) increases the quality bar for content, driving their main flywheel.
There's a reason why Threads was only conceptualized after Musk acquired Twitter and started talking about building an everything app. Pre-Musk Twitter was very content with its market and so wasn't a threat to anyone else. Musk's X was a different story (if he could execute on it, which at the time seemed more likely than it does now).
Twitter getting rid of 3rd party clients (pre-Musk), and shutting down API access (post-Musk) burned a lot of goodwill, leading to large spikes of growth of the Fediverse, and creating a window of opportunity for a microblogging service from Meta.
I don't think Meta would have been so keen to launch a competitor if people weren't so desperate to leave x (nee twitter) for another platform.