The notable difference is that they don't have a monopoly on violence

Most premodern states (at least) in Europe didn't really have a monopoly on violence in the strict sense either. Kings were constrained by a variety of laws and customs which they couldn't easily change without triggering violent opposition and had to co-opt the local elites (who usually acted as independent actors) whenever they wanted to impose their will outside of the area they directly controlled. Even an 'absolute' ruler like Louis XVI had very limited direct power compared to many modern governments.

Fair enough, it seems that you are primarily attacking the word Monopoly. What if we were to soften the claim to dominance. Rulers always maintained power based on their ability to resist violence from within and without. Economic leverage only existed as long as one could protect themselves from violent opposition. A trade embargo meant nothing if the opposition can seize your assets and production by force.

In today's world, mega corporations have no defense if a government actor decides to use force against them.

This utter defenselessness against violence is the main difference between a modern corporation and a feudal actor.

I feel this is only a partially true, at least in democratic countries governed by rule of law, because the threat is mostly only theoretical. While modern states are significantly more powerful they are generally governed by more rational actors who (both because of that and better access to information) are much better at estimating the consequences of their actions compared to medieval kings or other autocrats. While the government could go ahead and take all the assets of Apple, Google, Facebook etc. both the government and the people in charge of those corporations know that that's extremely unlikely to happen, especially in a violent way, since the government has very little to gain and a lot to lose by doing that. So while modern corporations are technically utterly defenselessness against violence as you say, arguably this is by choice. Unlike feudal actors corporations have no reason to invest any resources in changing this since they are in a much safer position.

I think I totally agree, and see this as a substantial difference between corporations and feudal lords. They have entirely capitulated on force and are comfortable in this position. You say they only maintain this position by choice, but what choice do they have otherwise? It is hard to imagine Apple, Google, Facebook having any success if they changed their mind and decided to challenge national governments like US or China with direct force.

It has pros on both sides. MAMAA wouldn't have their profit margins if they had to fund a private security force to enforce their contracts and protect their assets.

They get the freedom to focus on their business, within the bounds of the law, by ceding violence authority to the government. And in return, the government establishes and enforces those laws.

Good point. See: ARM China