We may see a day when the internet is not available, or when interacting with it represents an unacceptable risk. It's a good idea to know how to set up your own.
Jamming is done in military scenarios too, but in that case it’s limited by the fact that a jammer is a big transmitter painting itself with a big sign that says “fire missile here.” Civilian mesh doesn’t have that fallback.
The mesh networks are dealing with, by definition, a sparse and widely distributed set of devices which are independently configured and controlled, and in their current widely available form are only dealing with terrestrial communication. Without that point of centralisation you would need to focus on targetted regional jamming, as from a practical standpoint you cannot perform wideband RF jamming over an entire country - signal jammers don't scale that well, and geographic features come into play. As an example you might effectively block mesh networks from operating reliably in a given city, but if people were to move outside of that area then the mesh would operate again. Geography is both a strength and a weakness here: a mountain range will impede direct communication with someone on the other side, but it will also have the same effect on jammers which will vastly increase the cost to deploy them in a ubiquitous fashion.
The protocol-aware class of attack you describe does require some knowledge of the radio parameters being used, since LoRa runs on very narrow bands and uses both time and frequency-hopping to avoid congestion on any one virtual channel. They even apply (very basic) encryption to messages to prevent unknown senders from flooding the channel.
Unfortunately, both systems come preconfigured out of the box to use a default configuration which most users never override. So like cheap FRS/GMRS walkie talkies, all it takes is a few jerks who don't care about common use to overwhelm everyone with bogus messages. If you fire up a new device running the default Meshtastic firmware in any kind of dense urban environment, odds are it will more or less immediately get inundated with spam: "ping", "test", "hello from <neighborhood>", etc.
And since MT + MC both flood the shared channels to push messages across intermediary nodes, they pretty much self-DDoS by doing...nothing.
Also, it might not be jamming. It might be that whoever is operating the satellites at the time denies access unless you enable inspection, and then sells that info to somebody who would hurt you--or whatever other can't-trust-the-middleman dystopia you care to imagine.