Heh, my talk about being 'forced to play on adept' is more of a dramatization than a talking point.
Still, this is one of the main reasons summons are considered so overpowered and why they do seriously trivialize the higher difficulty levels.
I play a mage who relies on summoning, because obviously we do get 1-shot by pretty much everything, but using summons takes things too far in the other direction.
If i'm getting this, and it's not easy at the moment, he's saying all summons/npcs get a HP buff, but the npc's don't get the damage buff. So your summons are likely to survive longer when playing on a harder difficulty, but you yourself are not.
On master all NPC's get a HP buff and a damage buff when attacking you and only you. The NPC damage buff doesn't apply when NPC's attack your summons and companions, though, so they become pretty overpowered tanks compared to what they should be on master.