The latter. They rapidly prototyped many different compounds of different length, starting with only 12 amino, and then tested against 23 amino acids chain, to measure how strongly two molecules bind together. They found longer chains to be stronger bound to ACE2 receptors of CoV-2.
Their benchtop flow-based peptide synthesis machine can form linkages between amino acids in about 37 seconds, and it takes less than an hour to generate complete peptide molecules containing up to 50 amino acids.
Wow. Big fun.