they say they are #2. I thought Shopify and Woocomerce were #1 and #2 and they were a very distant third, or even fourth if Magento is to be believed. Or does WooCommerce not count as SaaS?
They do note later in their document that their competitors include Magento, Shopify, and WooCommerce.
I almost scrolled by and didn't care except for your comment, which surprised me quite a bit. The number of Shopify, WooCommerce, and Magento sites I've had to do some amount of work on over the years more or less tracks with BuiltWith's stats. I've never worked on a BigCommerce site.