By '5 years behind' do you simply mean that road rap is using the same sorts of sounds US rap was using 5 years ago, or that current US rap is actually an evolution of 2013 trap? I'm not trying to be critical of this notion, I'm just interested in how US rap has evolved sonically.
In the case of rap music by the likes of Playboi Carti, e.g., the beats are inarguably the most important element of the music, since the rapper is reduced to doing little more than ad-libbing.
Like I said, I can't comment either way, but I have heard road rap beats (e.g. 'waps' by 67) that were doing something different to american rap, and the flows are definitely different.