pukunui
Legend
I haven't specifically looked through the whole book yet. The lich's Spellcasting action indicates it uses Intelligence as its spellcasting ability. So the book is telling you that when it casts Counterspell or Shield as a reaction (or casts Clairvoyance while in its lair), it uses Intelligence.I wonder if there are any instances when it isn't the highest of the three spellcasting stats. I'll bet a blanket rule could have been made that monster spells are cast with the highest of the three.
It's possible there may be some monsters in the book with multiple spellcasting abilities, or this is just future-proofing for that eventuality, or it's worded that way in case you want to have a lich that uses Charisma instead (like a sorcerer-based lich?) ... but it still feels unnecessarily wordy.