The one thing that’s certain is that the 2/21 is a two ply, so no it’s not the same as the singles listed on their website.
Asking their advice is certainly best, but as I’m trying to get my head around all the different measuring systems too, I’ve done a bit of work on this.
The single at 4.8nm would give you 4.8 * 1000m of yarn for every kilo in weight.
I though it likely that the 21 is a worsted count, 21 being the number of 560yd lengths in a lb of yarn. It relates to the singles, and the 2 tells you it’s got two plies, so if my maths is right, one pound weight of this yarn should give you 560 * 21 /2 = 5880 yds. However, that’s very fine and would be considerably more than 15 wpi, so either that’s not a worsted count or my maths is wrong!
I googled and discovered that Shetland yarns were spun
to a Gala count (Gala being short for Galashiels, where the wool sorting depot is). Gala count is the number 200yd hanks in a pound, so if I adjust my sum accordingly, it would give 200 * 21 /2 = 2100 yds in a pound weight of yarn. (=4265m in a kilo.)
The article I found said that Gala 2/21 is the weight of Shetland 2-ply jumper weight yarn (which is equivalent to a 4-ply knitting yarn, as if you weren’t confused enough already), and the New Metric count is 2/8.4nm. So from that and my calc on yardage, it’s a little heavier than the 4.8Nm single.