100% correct. Even with fully patched software/OS/hardware/drivers/firmware we are regularly 'surprised' by a number of 0-day vulns that has been around for 1-2-5-10 years and 'suddenly' we discover it/them.

Putting my tinfoil hat on, I'd say that they got a new-improved way to get in, and they patch the old one because now the 'others' got whiff of this (e.g. shadowbrokers) and are about to start abusing it themselves :)