to be fair SC have a pretty comprehensive size guide on their website. I’m 5’10 which is right on the upper limit for a medium according to the SC guide (Mk1 Nomad in my case) and mine fits perfectly to me, but then Ive settled on ~22.5″ toptubes as being right for me. My previous 17″/medium Kona Coiler had it, as does my 16″ On One 456.
Plenty of riders my height ride the large and like it. TT length is more appropriate to measure a frame fit than seat tube when you have a foot of seatpost to play with rather than at most maybe 2 inches of combined stem length/saddle offset. Standover height is also less determined by seattube length with swoopy top tubes (pretty sure the medium and large Mk1 Nomad had the same standover height, with the large having a swoopier top tube).