@igor I spent some time in the past also trying to understand how it works.
That's exactly what it means, shinyapps.io makes the resources available to you as you need, which, somehow, is the same as giving them. Then it will consume the hours whenever it is used, like you just said with this 15 minutes time gap.
You're interpreting correctly, but think otherwise, you're thinking of a single user, with 15 minutes spacing each use, but in practice what happens are multiple users at the same time, and they tend to stay in the same 8 working hours range, and if there are many users at the same time, it starts getting interesting regardless of how many concurrent users you have, it only takes the same time. Of course there is processing limit, but it's not as bad as it sounds, try it!