Adding Monsters to Combat Against Yourself

[Blazinjsin]

There are only 2 monsters I saw that open a seal before combat, Destroying Angel and Great Cthulhu. The angel only opens one before combat if there isn't one already open, so I'll assume the monster in question is The Great Cthulhu. Since he says to open a seal before combat, the seal opening would not trigger if he was added to the combat, based on the ruling you just quoted.