Same. They managed to fake me out for quite a while.
Then again, I only interacted with the site by following links on sites like this, reading the linked item, then closing the tab. My entire impression came from the quality of those posts (took time to see bad ones make it to the front page of HN, for example) and the post page design. If I'd ever clicked around the site or visited the homepage I might have figured it out faster.
These days I expect a level of quality slightly better than Tumblr when I see the medium.com domain name in a link, and am occasionally pleasantly surprised. The image/marketing only went so far.
I would have to admit this was the case for me as well I didn't really pay much attention to it always thought it was the "print" version of VICE or something similar (and yes I know VICE is technically one or well at least started as a magazine).
I actually thought that Medium was part of VICE media for some time mostly because anytime I got content linked from it was from Motherboard or something else affiliated with them.
(I guess in a weird way that's what blogging is?)