Molding compound seems suspect, but it looks like high-CTI compounds are indeed available, up to 600 but maybe not infinity. The dominant component is silica fume, and it appears a wide variety of epoxy and phenolic resins are also involved. Basically if an arc forms, it can burn away the resin, leaving a huge amount of silica "ash" that hopefully tends to insulate things. This does assume the resin doesn't carbonize under all that silica, which I expect is why it's not usually more than 600?
Tim