That's a very clever trick.
It does the "mapping" by filtering the list (with the Set Analysis) and on top verifies that there is only one possible value (with the only() function).
If the table really has 1:1 relationship (meaning in the example that for every week there is only one month) then any functions is possible .
The only correction is: I don't use the literal quotes around the variable $(vSelectedWeek).
(I also usually don't use the $ in the Set Analysis expression)