They are not referring to a WhatsApp specific UX issue, but to the cognitive load of having multiple apps that you have to remember who to use which for, and their different interfaces.

raises hand I have a bunch of problems with WhatsApp.

Have you tried to restore a backup? You cant, unless it is uploaded to google cloud. No google account, no backup. (including the adress book that's tied to the account, since you are asking, they changed restore rules recently)

Have you tried denying adress book access? Whatsapp barrs you from starting a conversation. But there is the workaround with https://wa.me/+phone ...except for WhatsApp web

Have you ever tried putting Whatsapp in an Android work profile? Now try to export a chat!

Every once in a while a get the task to save all pictures of a conversation and it is usually a pain (If you think its easy, try again in Androids work profile).

From a UX perspective I would never mourn WhatsApp