Yeah same! So hearing this I was a bit stumoed at first. But like many many things as Gaeilge it's just a feature of another dialect.
I think you're right, but I think it might be common to add bomaite after the first six, for six minutes.
It's probably more common to add nóiméad - I think bomaite is primarily a Donegal expression, (and the FGB entry for bomaite points to nóiméad).
On the other hand, the exercise uses i ndiaidh and nóiméad might be more commonly paired with tar éis, though both are used (like "5 after" in US English, and "5 past" in Ireland).
But you're right on the basic point that "minutes" would usually be explicit in Irish, rather than implicit as in English, whether you say sé nóimeád tar éis a sé or sé bomaite i indiaidh a sé