You are right and Atalanta really took advantage of this opportunity, 5 goals can add to Atalanta's total goals, in the standings, even though Atalanta has the same points as Lazio, Atalanta managed to take 5th position because they have more goals. Atalanta is also still competing with AC Milan to get a ticket to the Champions League next season because the two teams are only 1 point apart.
Atalanta managing to win with lots of goals at home against a weak team like Frosinone is a very normal thing and Atalanta needs to make good use of it to get a better position. However, in terms of points competition, it seems that you have made a mistake in this case because the one point behind Atalanta currently in the top four of the standings is the Fiorentina team, not the AC Milan team which is still quite safe in third place in the current Serie A standings.