Hype works with absolute positioning. Hence, you can't expect reflows, as each rectangle isn't aware of the other. I ran through your example and font size became pretty stable with fixed line heights and a font selection that is guaranteed to be similar across devices. System fonts usual are not. Hype offers a flexible layout system for alignments and proportional positioning and sizing. But again, the rectangles don't really influence each other when using it. You can always break out of absolute positioning by managing a subtree of elements yourself in a rectangle (div container).
For other scenarios, Hype might not be your ideal solution or if you want or need to make it work look at observer-based solutions like resize observer. Also, this might be interesting for you: