Yes, it's counting the number of shields (achievements), we agree. And that doesn't seem entirely unreasonable only because the "Bot Challenge" also counts the number of shields, for lack of a more suitable method.
However it's jarring when the messages we see most often highlight the number of wins, not shields. And the shields display the number of wins as well. And the help text for Longest Series says it is for the number of successive wins.
The designer may have made the best choice under the circumstances. I even think it was rather clever. Yet it doesn't make it less confusing when the screen shows my "Longest Series" as 6 of 8 while two other areas of the screen tell me it's already 8.