Doesn't that apply to any in-ear headphone?

Other earphones, you can add a formed rubber cover to make them fit. Due to the AirPods charging in their clever snug case, you can't easily use them with any kind of cover attached.

More so for EarPods because they don’t go into your ear canal. But yes it does apply to any in-ear earphone, except those with custom molds. Eg Etymotic

Unfortunately they don’t fit my grandpa who has unusually large ear openings.

People with larger openings can put a little rubber on them to increase the diameter and add friction. There are some third party products and DIY methods out there.

As a sibling points out, this doesn't work as well for the Airpods which charge in a snug case.

Happily they fit my ears perfectly, but nothing 'one size fits all' actually does.

They’ll still fit in the case and charge. If the ears are truly gigantic, I suppose removable tips would be needed, but they just need enough that they won’t fall out. I agree they don’t fit everyone perfectly, but the weighting as it sits on the ear helps.

AirPod hook attachments https://www.amazon.com/EarBuddyz-Airpods-Attachment-Earphone...