I dunno, Stamos pretending to play the guitar or Brian pretending to play the piano.... not a lot of difference there

Brian can get away with it because he is the composer of the songs..I'm sure an elderly Beethoven may have gotten away with just sitting at the paino as well...
Not in those days, he'd have had the sh!t kicked out of him by an angry mob, all demanding their money back for the fiasco that was the Ludwig Van Beethoven Presents Piano Concerto No. 5 Live Tour.
But seriously, he was deaf as you know, and had to give up performing his music, because he played the soft passages a bit
too softly.
Also used to compose with a piece of wood stretching from his mouth to the piano soundboard so he could feel the harmonies.
Probably didn't shake his fist at the heavens when he died though, though that would have been a super cool thing to do.
And while we're destroying muscial myths, Henry VIII did
not compose Greensleeves. And when they try to teach my daughter this inaccuracy, which they have done for generations, I will march into the school and kick up such a fuss they will strike the Tudors from every junior school syllabus from here to kingdom come.
Pedants Revolt!