"Arms sold 1.18 million copies between its June release and late July."
https://www.nintendo.co.jp/ir/en/finance/software/index.html
As of June 30th, Arms sold 1.18 million in 2 weeks. It launched June 16th. This means that even if Splatoon 2 theoretically killed Arms legs it doesnt matter because the information theyre using is from before Splatoon 2. Essentially they're saying that Arms sold1.18 in a month and a half - when it sold that in two weeks
This is a case of kotaku literally looking at the day sales articles about Arms were posted, and not reading when thr sales data was collected. An all new low for gaming journalism.
Most of the early commentors seemed to have not read your post. I will say that I am a bit concerned Splatoon 2 will eat up Arms legs after it has released, but I think it's fair to assume Arms hit 1.5 -
1.7 million before Splatoon 2 came out, considering it had almost an entire month left