Standstill is a weird example because like poorhouse it's something that's really powerful by default, and then works as long as you DON'T do something. In practice that might turn out the same but for semantic purposes and when you try to have discussions it's not a good starting place.
Divining Top + Counterbalance might be a better example of the same sort of thing. The only reason that isn't called a combo in MtG is because it just doesn't cut the minimum requirement of how powerful an MtG player expects a 2 card synergy to be before he will call it a combo. Teferi, Mage of Zhalfire + Knowledge Pool is very similar in nature but dramatically different in magnitude and almost everyone will call it a combo.
So in summary, reasons not to call something a combo:
1. It's not that strong. Same way you might not choose to call the New York Giants 7th in the batting order a "baseball star". This is inherently subjective, and can change over time and from person to person.
2. Card Y is barely helping any more than some extra Card X spam would. In this case it's a strong monolothic strategy or strong card in general masquerading as a combo.
Not reasons not to call something a combo:
1. Some other thing is necessary, like a village or +buys. That kind of extra step ok as long as you are able to generate a dramatic amount of some kind of resource.
2. There is some way to counter it. That doesn't make it not a combo. Any play strong enough to make counterplay necessary is naturally, in and of itself, part of the "play" and significant. Oracle + Brigand shouldn't be overally poo-poo'ed.
3. The tempo is rush, or the tempo is slog, or the tempo is vp loop or long game (vineyards), or the tempo isn't any of these things. Combos show up in a variety of tempos, and some of them only thrive in a certain tempo. It's still good to understand the strong synergy and why you might want to force a certain tempo or unforce a certain tempo to stop it.
That's something people often argue about in MtG, since in eternal formats, combos are so fast and explosive that they kill on an even faster timetable than aggro and accelerating your combo becomes more important than deterring enemy aggro. In standard formats, combos often aren't fast enough to be that way, so they appear within a control shell and appear alongside lots of controlling elements.
In Dominion likewise it's not good to confuse a golden deck that would value an attack to slow down enemy progress while it gets setup to not be a combo just because NV bridge is so fast it should never buy anything without Bridge or Native in its name.
