I believe the poster specifies that the tank has two HOB (one a mini), period. The overflow mention was in response to a possible future project involving a sump. And I routinely use canisters on 20 gallon tanks for bio-only, plus 1-2 mechanical filters (but I use internal canisters rather than HOBs). Not your style? Not a problem, but it is my style. Again, personal choice item.
I also have no issue with anyone experimenting with various filter types (it is a good learning experience), and even less with using these on established tanks with other filters. Then, if there are problems, existing filtration can continue handling the load. If the new system goes as planned, pre-existing filters can eventually be moved elsewhere as needed or desired.