People don't avoid landscape on mobile because most sites are broken but because that orientation doesn't make sense for consuming website content (except video).
Apart from Netflix and video content, phone operating systems are mostly used in portrait too. So if you're checking email, using the phone or another app, etc. and then go to a browser, you'd have to rotate your phone to get into landscape mode. Even if you optimize it for landscape, it's highly unlikely to be a better experience over portrait. So most people don't do it.
Last time I looked at the overall mobile numbers, landscape and portrait were pretty close as far as usage. But that's including Netflix and other streaming video applications.
When they talk about how much usage native mobile apps have over mobile browser apps/sites they also don't exclude Netflix. That use-case skews the data.
My theory - people that use landscape mode on mobile browsers are above-average consumers of porn (or video in general at least). :)