That really is Johnny Flynn playing the violin in this episode. He is a talented musician in real life and has released several albums.
Johnny Flynn (Young Albert Einstein) & Shannon Tarbet (Marie Winteler) also worked together on Beast (2017) as Pascal Renouf & Polly respectively.
First appearance in the series of Seth Gabel playing Einstein's friend, Michele Bosso. Gabel is son-in-law to Ron Howard, one of the show's executive producers (he is married to Bryce Dallas Howard). Gabel would also go on to play a completely separate part in the second season of the show which focused on Pablo Picasso.