I have never had the need for pure milk foam until now.
However, since only coffee with milk foam is available via the menu selection, the statement "Not possible - only milk foam" is correct.
The only possibility is to quickly change the cup before preparing the milk foam, but you didn't want coffee.

Simply pour the desired amount of milk into the milk container and select the compartment for ground powder. As there is nothing in there, only the milk foam is released.

not possible !!!! correct