i didn't do in depth analysis, but heres my thoughts:
First of all who really cares if this guy thought you were sandbagging at the end of the day only you will really know that and have to live with it, but i agree i don't see it either.
Just the simple fact that you were playing a match play game against a higher level knowing full well it would drop your average if you won sort of points to the exact opposite. I think a sandbagger would be playing same or lower levels and losing on purpose so as not to lower score and not tier up, especially considering a tier up for you brings new tees.
Seems you've been around for a fair amount of time and certainly have equipment and scores that seem commensurate with your level and tier
sounds like he was just pissed he lost