IMHO "for a belt" and "as a belt" have almost identical meanings and could be used interchangeably. However, there is a slight, nuanced difference between the two. To me, the way they are used here, "as" means in a similar way, and "for" implies in place of. "For" can also be used in a way that implies "with". Consider "this part is used for a car" as opposed to "this part is used as a car". So, "for a belt" could also mean with a belt or as part of a belt. ... All that said, "for a belt" and "as a belt" both make sense to me!
Any one of these should have been accepted: "This is/That is/That's/It is/It's". If it wasn't it is a technical error and should be reported directly here:
Thank you for your assistance.