Their sizing page would imply that is indeed the case. I ordered S knit and M shirts... I'm a perfect 48 in Cabourn knits and jackets (I don't own any Cabourn shirts) so I'll let you know how the comparison works out if you're desperate to know.
MoK might have better insight.
Only the jacket has arrived so far and based on that size up! XL seems to equate to a very slim 52... Quality is good though :-)