Meta doesn't really offer any profit-making AI product, so I don't understand what commoditizing it buys them in this narrative. You could argue they rely on AI and so want it to be a cheap commodity - but that is not harming their rivals, that is just a different way of stating that Meta is engaging in something that enables people to collectively work for mutual benefit, which is not a nefarious plot against rivals, it is laudable cooperative behavior.
It is a business decision to open source his models, because he has been behind the curve from the start. Do you believe if he had chatGPT, he would open source/weight it? I don't think so, he would treat it like Instagram.
The point is, AI/LLMs are seen as the next frontier. If one of the closed ones becomes dominant, then that company can use its user base to attack Zuckerberg's platforms. That google failed with google+, does not mean the next competitor will.
A telling sign is how he integrated his AI into whatsapp. It seems out of place to me to have an LLM integrated into whatsapp, but he desperately wants a part of the AI cake.