Your title is misleading because "Wordpress" as a general brand encompasses the open source Wordpress.org CMS workhorse, which most of us on HN typically associate with the name. Wordpress.com the cloud blog platform has lots of valid competition, notably Tumblr and Blogger.

I enjoy learning the bootstrapped rags to riches story about Squarespace, but your title frames the story as if its about a new competitor to Wordpress the open source CMS, and it is not.