That was at one point true, but hasn't been true for at least a few years.

Heroku could have gone back to that i suppose, instead of getting rid of free dynos entirely, but they didn't.

Still... I think you could basically only run one free dyno per account 24/7. I am curious if they somehow fit their entire production in one free dyno, or were somehow splitting things between multiple accounts, or what.

I have several legacy free tier dynos that don't have the restriction. They run 24/7, at least for another day or so.

If I'm not mistaken, if you gave Heroku a valid credit card, it would add a few hundred extra hours for your dynos.

that's correct, you could be have it on 247 with trick!