I can understand that you are basing your interpretation on the following quote:
If you target an object or creature that is the effect of an ongoing spell (such as a monster summoned by monster summoning), you make a dispel check to end the spell that conjured the object or creature.
But, it seems apparent to me that this sentence was written without taking into account multiple monsters summoned by the same spell. For one thing, the "object or creature" portion of it is singular. It in no way talks about multiple monsters summoned by the same spell, it just happens to have wording that would wipe them all out, apparently by accident.
I suspect that the designers never even considered the question. However, they did consider the question with Antimagic Field (or any other area or single creature target spells) and the entire spell is not surpressed, just the monsters within the area.
The fact that area dispels on area spells where the point of origin is outside the radius of the dispel also works this way is also an indicator that only targeting a single summoned mosnter (or targeting all of those within an area) is what is supposed to happen.
Granted, you are correct with a literal interpretation, but I think that sentence is an accident. JMO.
It seems very odd to me that any other spell which catches some of the monsters within its area only affects those within the area or any other spell which targets a single monster only affects it, but Dispel works counter-intuitive to all other such spells. There is no need for a special rule here, hence, I think it is an accident of how they worded it.