The False God power in Realms of Power: Infernal, His Master's Voice, seems weird.
It lets the False God summon demons, spending 1 Might Point for each point of Might possessed by the demon being summoned. However, based on the way Creature Powers work, the Penetration of the power would be reduced by 5 for every point used.
None of the demons in RoP:I have the Penetration ability.
Of the four example False Gods, the two Might 10 ones cannot summon Demons above Might 1. The Might 30 and 50 examples cannot summon above Might 5 (aside from the hypothetical existence of a Might 8 demon, which isn't really important). Beelzebub, theoretically Might 75, can summon Might 10 demons, or perhaps Might 15 if he's got some Penetration and a good arcane connection.
In Transforming Mythic Europe, Rumael and the Lady of the Purple Bower are both presented with this power, and their minions are all Might 5, which they could actually summon, since they have Might 40 (and, again, no Penetration).
So, is this intentional, y'think? It feels a little off to me. Not like, terrible. But odd. Like, Adramelech, Demon King, can't summon Lickspitten, Cat.
If the False god knows the true name of the one summoned, it gets a lot easier. Big boss demons like Beelzebub and Adramelech should know the true name of lots of smaller demons who serve under them in hell.
Demons don't seem to have the Penetration Ability, though. So, a True Name doesn't have anything to multiply, right?
Entities missing abilities they should have is very common in Ars Magica, and even more so in RoP:I. Normally if t does not affect the way people expect to interact with the character it is simply left off. Meanwhile demons who are centuries old have fewer abilities than a 20 year old grog. I would say it is almost certainly an oversight, though I think it could be played either way.
For the oldest and most powerful demons, you are looking not at centuries but millennia in age. If a big boss demon who has been operating for thousands of years and has the His Master's Voice power does not have the Penetration ability developed to some extend that is just plain odd.
If you are stating one up for your game it would be your choice if you wanted to include points in Penetration. One that is a summoner would greatly benefit from this as you have noted and you could also decide that they have the true name of all their minions. I would suggest not taking it to far though, since a very powerful demon which can summon hoards of powerful demons can easily become an unstoppable threat.
'Demons are written improperly/incompletely' is a fine answer, I guess. I was curious if there might be an alternative interpretation or an unseen clarification, but maybe not. Realms of Power: Infernal seems to have many oddities about it.
Keep in mind that arcane abilities, except Parma Magica, do not require the Gift or any supernatural virtues to use. The only virtues are ones which provide access to restricted information. as such any demon is capable of learning penetration to the degree that a demon is capable of learning anything (demonic gains in abilities is not addressed in the text, presumably the greatest limit is sloth). As such it is certainly possible for a demon to have the penetration ability.
At the same time it is understandable that a SG doesn't want to write out 10,000 xp worth of abilities for a character who likely will only have incidental "screen time" compared to those who travel with the party.