This isn't the moment he was flagged offside.
I've watched the same piece over about 4 times myself (Not a chance of knowing for sure from the South stand) And I'm pretty certain he comes back from an offside position to challenge the clearance/pass.
I shall be delighting in a little more of my (early) retirement bottle of spiced rum, gifted to me by work colleagues the other night, Daz.
I'll probably do this later on whilst checking out next season's opposition on MOTD2.