Glycaemic Load is completely variable dependent on serving size. In the testing carried out by Oxford Brookes the subjects consumed 100g of Huel to 500ml of water, the recommended measurements.
The GI of Huel is low, yes. However this is comparable to ‘meal replacement’ products - N.B. We do not consider Huel a meal replacement, but it is the easiest way to compare them. In the International table of glycemic index and glycemic load values there are many products that are likely to be of a similar nature to Huel. For ease, here is the section I am referring.
I expect, a combination of low GI oats with low sugar and the high fibre content are responsible for this. Soylent is the only product I know off the top of my head that have done GI testing too (do link others here so we can all see though!). You can see here that their GI is not as low as Huel. I expect this is due to their higher levels of maltodextrin and sugar and lower fibre levels. However this is just speculation.