If I understand this comment correctly, you seem to be implying that "their" works because it can substitute as a singular pronoun meaning "his or her".
That's not quite true. What makes su singular here is the fact that it refers to one chair, not the number of people to whom that single chair belongs. If there were more than one chair, sus would be correct regardless of the number of people involved.
This holds true for other pronouns: mi amigo, mis amigos; tu gato, tus gatos; etc.
"Su silla" would work better "la silla de él" works too and means the same thing but literally it means "the chair of him" which sounds a little weird. So, "su silla" is a better option. (By the way, there's an accent over the last "e" in your comment, without it, it would mean "the" (masculine))
They should fix a lot of the enunciation, since I've gotten things wrong because of it. You should be able to go to your "settings" page and turn off the speaker setting. I also recommend turning off the microphone setting, since the website isn't very good at picking up people's voices well.