I think that phyllostachys dulcis is a possibility considering the factors that it has big leaves, puts on a huge upsize, branches really low, appearing to have short branches, spreads very far with few culms and is shooting at the right time if your temperatures have been identical to those in south carolina where bamboothew is. Mine generally shoots well after the nigra, aureosulcatas, but just a bit before parvifolia, and about 2 weeks before atrovaginata which comes last.
One of the ways to make sure is to check if the leaf buds are elongating early. Dulcis leaf buds are the first to activate here with the onset of spring. Another way is to show a close up picture of a shoot when it is still under 1ft high, revealing the sheaths when they are the most distinct. A close up of the new culms toward the bottom would also show the distinguishing features of dulcis. Another way is to look at the rhizomes. They are generally short noded, hose sized, and white.
Here's a picture of mine taken 1 month ago. The only difference due to my climate/growing conditions is that mine barely spreads, but otherwise, it has all the other distinguishing features of dulcis. The leaves are huge, culms are flat green and very smooth, internodes and branches sometimes have a slight bend, branches are short and point slightly upwards, branches start almost near the bottom of the culm, and the white rings are very prominent. If your bamboo has the identical features as what's shown in this picture, then you might have dulcis.

Here are a few more pictures of dulcis which might help you identify if you indeed have the same plant that Bamboo Gardens claims to be phyllostachys dulcis.
Shoots-1ft or less


Airlayering shoot division-showing new rhizomes coming off bottom nodes of culm


Entire clump taken from a good distance
