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The problem is at a larger scale than you seem to think though, because we are specifically talking about ADDING potential entities to the already existing system. I definitely agree with you that 'this is what stress tests are for', and personally believe they will achieve very robust network architecture that will stave off some of the problem. That does not, however, make the concern invalid.
For example, full party of necromancers, who are around for siege summons is 48 entities. Even if we reduce the pets and mounts out of the equation that's still 40-56 entities for one party doing PvE depending on the enemy population. Probably tolerable in most situations. But sieges are a whole other beast.
There is going to be very high population caps for sieges, however, and the numbers start scaling quite a lot. 100 necromancers? 400 entities for one team, being generous assuming it's a 100 man siege and again ignoring pets (you can't ignore mounts in a siege unfortunately but it's a lot more volatile so let's exclude them.)
That's not even accounting for the enemy side. Intrepid is aiming for at LEAST 200 v 200 as a design goal. Being even more generous assuming none of the enemy are summoners or have battle pets and only half of one team is necromancers that's 700 entities. And again that's give or take another possible 200's worth of mounts.
Can I win a siege by gathering all the necromancers to lag the server during the siege window? Possibly. It depends how much combat is designed around certain high damage/cc moves being reactable and how beefy the network architecture is. But if lag matters, you have now incentivized this being a strategy and it therefore NEEDS to be a factor in design.
Either way, the more entities you add the less people you need to cause lag and the wilder the numbers become. Just adding 'three skeletons' like was suggested in the opening post isn't adding 'just two more entities'. It's adding two per person. In a 200 vs 200 siege that is, again being super generous like I was above, another 400 entities...
But wait it's actually 1200 because you can have 3 summons of 3 skelies. So that puts us at a very generous hypothetical 1900 entities+200 mounts. You have to plan your network infrastructure and combat design around this which demand trade offs no matter how genius their network engineers are. Do you see the possible technical issues and why it's a valid concern others may want addressed? We haven't even brought up siege engines, projectiles, destructible environments, or loot drops that could complicate this.
Even if we limited it down to just one group of 3 skelies that's still an extra 200-400 entities you have to worry about on top of what is already going to be necessary to account for to prevent 'The Necro Zerg' ever becoming a reality.
The true counter argument is ofc 'make those three entities just one entity' which is a valid approach. But it is debatable if that would achieve the 'horde' feeling if you knew deep down that that one entity of three skelies wasn't actually real and didn't have individual hit boxes (which as far as I understand is part of the appeal of a horde.) However, it is a far more acceptable compromise in my eyes.