I feel like even the phrasing of the original assumption that "we have more bank tellers now that we had before", which seems to imply that ATMs didn't affect or even boost the number of bank tellers is flawed.

If you look at the graph, the number of bank tellers from 1980 to 2010 went from roughly 500k to 550k (a 10% increase). However, the U.S. population grew from 220M to 305M in the same period (a 40% increase). To me, that seems to indicate that less and less people were becoming bank tellers after the invention of the ATM. Although from the graph again, you can see that the correlation is quite poor anyway.