You might be correct when it comes to English, but the Swedish sentence could not mean that.
If you'd want to express that someone dresses up as a moose in Swedish I would suggest;
A) to say "Han klär ut sig till (en) älg" - "He dresses up as a moose" or
B) "Han går som älg" - omitting the "en" in this sentence gives a different touch to it and, knowing the context, I would understand this.
Please tell me you did not just randoly come up with it. It immediately reminded me of this novella: https://www.goodreads.com/book/show/24077.Crabwalk