Why is it also considered correct when "peco" takes the accusative here? It's not conveying direction, so what's the deal?
It should not be correct for 'peco' to take the accusative case here.
Shouldn't "tio" be "tiu" in this sentence due to the sentence coming of as one talking about something that the speaker and the other party are both aware of?
I think you have to use "tio". You would use "tiu" if there were several things and you would point one of them saying: but this one (contrary to the other ones) is only a piece of plastic!
Tiu is only used when directly attached to a noun. It works the same with kio/kiu. Kio means “what” and kiu means “which”
Mi bone legis vian kurseron pri "peco de" kaj "peco da" sed en la supera kazo, mi emus diri "peco el plasto" ĉar la peco estas farita el plasto. Ĉu "el" estas eraro ĉi-tie?
Gee, Mr. Krabs, you're getting all sweaty again.