Lazio and Athletic Bilbao have the same points, namely 16 points with 5 wins and 1 draw, on the other hand both teams are also the teams with the strongest defense so far in the UEFA Europa League, at least from 6 matches. Lazio only conceded three goals, and Bilbao even less, they only conceded two goals. On the other hand in their respective domestic leagues, they are also teams that are quite reckoned with when facing top teams, especially Lazio who were able to stop Napoli's winning record and were able to hold Atalanta to a draw.
Lazio and Athletico Bilboa has been very outstanding in the Europa league games and they are sharing equal points and just like you said, their defense line is perfect such that they don't concede many goals like the other teams in the Europa league. Going six games without a lose in the Europa league games is something only few notable teams can achieve but among Lazio and Athletico Bilbao, Lazio is stronger and their attacking force is Superb. Lazio have been showing good performance in both the Serie A and the Europa league and despite that they don't have upper chance in their domestic league but i feel they can be up to something in the Europa league because the strength they have shown in the Europa league games cannot be measured with that of the other teams. We can't also underestimate the strength of Bilbao; they are doing great in the La Liga and Europa league, so they also deserve praises for their awesome performance so far.
Lazio and Athletic Bilbao are currently receiving a lot of praise after their impressive performances in the Europa League, both teams currently have a lot in common, they are both ranked fourth in the standings in their respective domestic leagues. One more win for both teams will ensure their progress to the last 16, I think they deserve to be considered as strong candidates to challenge for the trophy this season. The performance they have shown has exceeded anyone expectations, and maybe one of them will succeed in winning the trophy this season.
Learning from last season experience, the Europa League always presents many unexpected surprises. Manchester United and Tottenham who were initially more favored, have not been able to perform to meet expectations. I think the change in the Europa League format has succeeded in making the competition more competitive since the beginning of the season.