D&D 5E D&D Basic DM rules updated

[Kaychsea]

If you are using an action to kill one or more skeletons rather than damaging the lich, the skeletons have made a significant impact.

True, but not exclusively true. Area effect spells can take out the mooks and damage the BBG.

 

[MonsterEnvy]

If you are using an action to kill one or more skeletons rather than damaging the lich, the skeletons have made a significant impact.

5e encounters are short and deadly. Every action counts. Even if the minions are just soaking up a player action or two, they have contributed enough to warrant their share in the XP budget (which is relatively tiny, compared to the lich's share).

But by the default rules they don't do enough to warrant increasing the difficulty multiplier. By default if you included them with the Lich the encounters exp value for determining difficulty is 100,800. If you don't include them in the multiplier the difficulty is 33,600. The later is correct by the new rules as you still get xp for them but the weaklings don't do enough to warrant the multiplier increasing that much.