All the transistors will be the same voltage rating, yes.
(Which, by the way, means the PNPs are also rated for 18V
emitter-base -- because they're lateral PNP in that design, so are actually NPN collectors, touching close enough to get an awful, but usable, hFE.
)
The breakdown mechanism will be avalanche. Most chips of the era were modestly doped, giving a 30-40V limit. A heavier doping would give lower rating, higher leakage, higher current density and shorter carrier lifetime (faster), which is probably what was used for the 555. Gold doping would give very low rating, and more leakage and speed. (AFAIK, 74 series TTL was your basic 30V NPN process, with gold doping bringing it down to about 8V breakdown. 2N2369 might seem kind of odd, as it's a gold-doped switching transistor rated for 15V -- but if you suppose it would've been a 60-100V bipolar without the gold doping, this seems quite reasonable.)
Taking a quick look at several datasheets, I'm surprised I don't see any individual pin voltage ratings; a few specify input voltage limits (0 to VCC), but leave OUT and DISC undefined. Perhaps this was typical of the era, when equivalent internal diagrams were common, and processes were cheap and simple (all the same kind of transistor). Modern chips do not typically show internal diagrams, ESD diodes, and can have many dependent voltage limits (like a bootstrap gate driver's 18V supply limit
on top of a whopping 600V high side limit).
Tim