This is not incorrect. However, it may inadvertently suggest restrictions on the purpose already explained above:
...visitors or guests may go to these lavabos to wash their hands, ect, before sitting and eating the meal.
In fact a lavabo will offer (non-staying) guests,
whether or not there is a meal involved, bathroom facilities
during the extent their visit, precisely to avoid, again, their having to enter the more private areas.
Now this is additional interesting information, but it doesn't really apply to a flat:
... if their houses have a backyard ... in this case one will seldom hear the word lavabo, it's simply banheiro...
Once one strays from apartments, there are many more options in termos of location of a half-bath fulfilling the apartment lavabo function explained above.
Regards