You are correct, the sizes attribute is missing. This joins some of the other posts posted here about this specific issue.
Our next update will be retaining the sizes attribute and should allow the browser to select the correct source without having to modify the srcset.
I could be wrong, but I’m guessing it is because iPads, for instance, have such a high resolution that images that are twice the size specified look better.
And you could be very right
I actually read that browsers detect devices pixel density and automatically choose the right image dimension, which allows not to bother with 2x modifier.