Playing at steam? Then you can log in at aoe2.net with your steam log in (Note: This is a third party site).
Then you go to https://aoe2.net/#aoe2de-matches-my-recent-matches (only accessable if you are logged in). You can see your own history and you can also click on the profiles of your allies and enemies. So it is possible to know exactly who you are playing.
123456 is an account of vivi, a well know top player. It isnt the first time he is caugth as having friends on smurf accounts in team games. Vivi is indeed one of the best Chinese players. Based on the current rules it looks like this behaviour is allowed. The devs dont really seem to care about the smurf accounts. Also on the 1v1 ladder there are many smurf account. I know that mbl had at some point 3 accounts in the top 5 for 1v1s.
It is really ashame the devs dont want to baby sit and ban all the smurf account. If they dont want to baby sit, then they really need to implement something for the team games to limit smurfing in general. In my opinion this can be done by changing the way the TG strength is calculated by using some kind of weigthed average instead of just the normal average. Multiple suggestions to change this calculation are already be posted in the thread Analyses of the ratings - Spotting the issues. As result of a different calculation that puts more weights on a the higher rated players, the impact of smurfing will be smaller.
Currently: Vivi + 3 smurfs of 1k result in an average of 1.8k, so they will be matched against 1.8k players. But if the average calculation is changed, so the same team will count as something like 3.5k and will be matched against other teams that are 3.5k, then it will already limit the usefulness of smurfing by a big margin. Such ideas to change this calculation are already discussed in that thread i mentioned earlier in this post.