FWIW, some people have reported that having a single surround-back speaker centered behind them can result in a strange sonic illusion, such that some sounds which should be coming from behind seem to be coming from in front.
Otherwise, yes: 6.1 channel audio is designed to use a single rear speaker, while 7.1 channel audio has both of its rear channels mixed together into the single rear speaker. The type of discrete encoding is irrelevant. It's not "matrixing", though. That's the process used to blend multiple audio tracks into fewer in a way such that they can be separated out later. In this case, they're just blended together into one channel that'll never be separated.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped