Co-worker here, yeah, truly some pages of the site are not yet quite responsive, sorry for that, we plan to improve it in the future. On the site there are some particular templates designed to show the responsiveness or non-responsiveness, you could open them on smart phone or desktop computer to see the differences:

http://semantic-ui-forest.com/templates/bootstrap/grid/

http://semantic-ui-forest.com/templates/bootstrap/navbar-fix...

http://semantic-ui-forest.com/templates/bootstrap/non-respon...

Semantic-UI gives more freedom for features like responsiveness, theme customization with 3000+ variables, although the default configurations are also very good.

I think the layout issues go beyond not being responsive. I usually don't mind at all to just center on the main article text and read in the general layout. However, the article layout assumed such a huge screen width that it's impossible to read on an iPhone without coming in and manually dragging left and right as you read.