Read the faq at Whatsapp support site carefully:
"We don’t support setting up new accounts on WiFi ONLY devices."
It said WiFi only iPadOS devices but not cellular version (WiFi+GSM) of iPad with SIM card slot (there are two versions of iPad all along).
Thus with cellular iPad you still able to receive standard SMS message even from non-Apple devices. The only thing you can't do is to make phone call even with cellular iPad.
Anyway as other posters have pointed out, SMS messaging capability is not required for What's App as communication can always be routed through Internet networking just like in iMessage app on iPad.
Many early developers do not want to take trouble to unify user interface design in XCode development environment between iPhone and iPad (the base code can remain the same). Interface builder module in XCode (later versions but not earlier ones) takes care of differences between iPhone and iPad pretty well thus extra effort is not much. I am surprise that even Meta such big company does not want to do it (not profitable or incompetent ?).