But there's a $21000/mo maximum for the $275/mo, after which it is 2.75% for additional swipes. Purchases >$400 also are charged at 2.75% for all volumes.

So, it's as low as 1.3% if you do exactly $21000/mo in sales. At $10k, it's break-even. Below $10k, it's worse than 2.75%. Between $10k and $21k it gets better, and gets worse past $21k. (Since they include Amex, this would be great for someone doing $21k/mo in Amex charges, though...I think only Costco pays less for their Amex charges.)

Compared to running your own account (which is probably about 1.5% all-in for big ticket sales), which clearly makes more sense at some volume ($50k-100k/mo vs. Stripe, probably)

I read that as 2.75% below $10K, but I think hitting $10K in Square sales per month is difficult if we (generously?) assume that's 10% of your customers, so effectively it's like any other ~3% CC for a majority of small and local businesses. It's good to hear it's not generally worse though, I guess they thought about that.