I had this exact same problem, made out a ticket which was solved at the 2nd attempt.
The op is indeed correct in saying there was no information stating that you may only purchase 1 package per era, and as he said before you actually purchase the super starter package BOTH are indeed available,
Does this suggest that had we bought the era package instead of the super starter package at the start of the round that the super starter package would have still been available provided we were under 600k prestige?
Well according to the mods not,
Maybe it's something they'll decide to take a look at during one of these maintainence downtimes and completely eradicate 1 or the other from the gold shop rather than just greying out the purchase tab with some irrelevant text appearing talking about it only being available within 24 hours of the start of the era,