According to an older scientist, Anders Fomsgaard at the Danish Serum Institute, this is how they did it ”in the old days”. He is the father of one of the authors.
This eliminates supply chain problems for reagents and was shared quickly to help in Spain.
Preprint by Fomsgaard and Rosenstierne:
https://www.medrxiv.org/content/10.1101/2020.03.27.20044495v...