Qt module .pro files that declare QT_FOR_PRIVATE dependencies are meant to be transitive dependencies for consumers of the private module counter part.

In lay man words
qtwayland/src/client/client.pro content:

MODULE = waylandclient
QT_FOR_PRIVATE += service_support-private

That means that any other module that depends on waylandclient-private, also transitively depends on service_support-private.

QT_PRIVATE += waylandclient-private
# Implies QT_PRIVATE += service_support-private

So in CMake terms we would have to generate the following snippet for the wayland client project

qt_extend_target(
  WaylandClient
  PRIVATE_MODULE_INTERFACE Qt6::ServiceSupportPrivate
)