The key insight/ intuition (non-obvious) is that- if you did a worked example first- you'd find that the heap always contains ONLY 1 element from each list/ row array. The heap NEVER contains 2 or more elements from the SAME list.

This is important because it means that the range calculated at every iteration is always guaranteed to be a range that has exactly 1 element from every single list!

I think the description lacks this key detail, as it's just asserted that the stated solution works but it's not immediately obvious that it should.