Personally I have no idea why they rolled these theme styles for nautilus while they have been worked on Communitheme [1], which does not have tab-like menu [2]. However I'd like to note even in adwaita (and many other styles) there is design inconsistency between nautilus and gnome-tweak-tool. Sadly, Ubuntu dev team and community have not released any stable version Communitheme yet, so they were too late to bring new, consistent theme to 18.04.