Offensive lineman are expected to throw around some serious weight during the off season lifting regimen to ensure they're able to do the same come game day. That, on top of conditioning and a lack of contact (that all good offensive lineman crave), can make for quite the off season grind.
With that said, there is really only one real way for those big fellas to beat the grind of the winter routine - get the entire offensive line together at the indoor facility, and act like skill players for an hour or so with a legit 7-on-7 scrimmage. The extra conditioning is an added bonus.
From the looks of it, it's just what the doctor ordered.
Now, what they may lack in proper throwing mechanics and crisp route running, they more than make up for with surprising athleticism (see the one handed grab and check-downs by the quarterback to the deep ball) and enthusiasm.