It's true that that's another possibility that could happen, we haven't seen how Tottenham will play without Son. Even though I don't hope for it, if Son is injured or absent for some reason, Tottenham will have difficulty consistently achieving full points. On the Man City side, Pep squad has a habit of being hot late and looking more aggressive in the last half of the season, I predict they will return to stability quickly.
With Man United and Chelsea inconsistency this season, only Man City, Arsenal, Liverpool and Tottenham are in tight competition for the title race. Even though Tottenham are top of the standings at the moment, I think Arsenal and Liverpool are more worthy of challenging defending champions Man Ciy, and Man United and Chelsea are likely to finish outside the UCL zone this season.
If you look at the current performance of Manchester United and Chelsea, it is very likely that they will not be able to compete closely at the top of the Premier League standings this season to be able to finish in the European zone. I am really very pessimistic about these two teams.
However, regarding Tottenham, I think we should not underestimate them this season, the strength of the Tottenham team is quite comparable to Man City, Arsenal and Liverpool, so it is possible for these 4 teams to compete fiercely for the title this season.
Chelsea will find it very difficult to find a draw at home if the opponent is Arsenal, because as has been seen, Arsenal have started to be more consistent in winning matches. And in general, Chelsea's strength this season is still very far below Arsenal's, so it will be very difficult for Chelsea to hope for a draw at home when facing Arsenal at Stamford Bridge. Apart from that, what you say is also true that Arsenal will be more confident in the next match after Arsenal managed to beat a team that is quite strong this season, namely Manchester City.
It is indeed quite difficult for Chelsea to get points in the match against Arsenal, as we know that Arsenal this season is very strong and that is in contrast to Chelsea who so far have not found their best performance.
I also think it is very unlikely that Chelsea will get points in this match even though the London Derby match is played at Chelsea's home ground.