Does this amateur observation hold a record even including robotic searches for supernovae?
No, supernovae have been observed earlier and with better cadence than this one. 

The Kepler Satellite has been observing galaxies at a 30 minute cadence from months before the supernova explosion to months afterwards.  Most of these have been Type Ia which do not have detectable shock breakouts, but there was one SN with a breakout published in the Astrophysical Journal before this one: "Shock Breakout and Early Light Curves of a Type II-P Supernovae Observed with Kepler".  

The Nature paper linked in the question provides 4 other SN that were also detected about as early.  I think the new thing here is that it was discovered so early by an amateur astronomer.