Construing a copula with accusative case is a very curious and interesting mistake!
It's kind of Slavic-like construction.
Wait, this is a thing in Esperanto? That wasn't a performance error on Emile's part, but they actually use accusative case predicatively like Slavic languages do instrumental case? That would be pretty bizarre.
I just read this in the Wikipedia article on the Mattaponi River and it really tickled me. If only all language were so rational!