What you're missing is that the MM6 is stated as having an "easy chord function". I'm well aware of Yammies use of "phrases". This type of setup is used in the the Yamaha QY series for example.

I know the Juno series well too. Actually I know synths in general well. The reason I say "arranger" is because the MM6 has a button under the pattern control for "Fingered" which I would assume utilizes the "easy chord feature". It also has other arranger feature related buttons too.

Also take note that the text says "full backing" for "live" performance as well. I wouldn't be the least bit surprised if Yammie has the S-500 as the lower end arranger and also made another geared towards the synth line as well.