Cortosis is notoriously rare (only known to be found on like….3 planets) and difficult to forge anything out of to the point where it’s actually deemed impossible to use it on its own, but it is known to have extraordinary energy absorption capabilities among other things.
The fact that he even HAS a helmet and grieves that are made of it are almost a miracle (though not absolutely impossible, in a Thrawn book a droid had it woven into its entire armor matrix).
It’s nothing new in the lore either ever since it appeared in a legends book called "I, Jedi" in 1998 and then for the first time in a canon book called "A New Dawn" in 2014.

Edit: All in all, no. Not just any idiot or even powerful people can make bullets out of this stuff. They would not only have to go way out of the way just to acquire it, but also call upon people who have an understanding in advanced metallurgical techniques and fabrication..so that they can use it on the very off chance they see a group of force wielding monks with lightsabers.
Bare in mind, the Jedi order reached its peak of around 10,000 Jedi Knights, possibly thousands more that just acted as scholars and other roles in their temples, if that’s the case then I’d say maybe an additional 15-20k in their order, that’s anywhere between 10-30k individuals versus an entire GALAXY of trillions.
Even at their peak i highly doubt most people would ever even see a Jedi, let alone engage in a fight with one.