If they have another draw in that game, it would certainly make things quite complicated for them. It would still not lead into a deadlock, but it would make the situation dangerous for them as that would reduce their room for error close to zero. Since Lens has shown that they can win a game against Arsenal, there aren't many reasons to believe that they can't do it against Sevilla or PSV and that would make the group quite exciting for the fans and quite messy for Arsenal.
Arsenal's performance against Lens was not bad. There was some strategic mistake so they lost. Arsenal's overall performance in this match was very good. I don't think Arsenal will make the same mistake again. Arsenal team is strong enough. Sevilla's performance, on the other hand, has been weak and erratic. I don't think Arsenal will make any mistakes against this team. If Arsenal's defense can perform a bit better in this match, I think they can beat Sevilla. Sevilla team can be considered as the weakest team in Group B.
Arsenal was the stronger team, but these things can happen in any competition and to any team. Have a look how Manchester City lost the game against Wolverhampton Wanderers. It is an unexpected end result, especially when you watched the game and how Manchester City was in full control.
But all these teams in the Champions League are there for a reason and that is they can all score against anyone. Sometimes they don't and sometimes they score against teams that are far superior. For Lens it went more than smooth against Arsenal, but I was trying to emphasize that if a game early in the group stage goes like this, it makes it harder in the next games even as the group favorite because no more errors are allowed.