It's a matter of how many fucks the devs gave about doing quality work. You can find inexperienced people deliver quality work simply because they're passionate and they care (Half-Life was made in part by people with no prior game or coding experience), and you can find supposed rockstars deliver slop because for whatever reason they don't care, they just want to punch in, collect paychecks, then punch out.
FAANG stopped being a benchmark of skill a while ago. It's a skill of griding leetcode and 7 stage interviews to get it, then resting and vesting till you can jump ship to something better or get mass layd off.
That's not to say memory corruption issues are the only kind of issues, and rewriting a mature C/C++ codebase into a memory safe language may well introduce more issues than it fixes...
It's also had a pretty 'normal' (read: higher) amount of non-memory related bugs such as SMTP smuggling of filesystem permission TOCTOU, so it's not like its devs are somehow really good at just not writing bugs.
Maybe they have chosen a software architecture that allows them to avoid complicated memory-related logic? I genuinely don't know, would love to hear from a postfix developer.