The paper does not use sidecoin or sxBTC. They use sidechain coin and bitcoin. They try to say that it remains a bitcoin while it travels the sidechain. I guess that could be called a sidecoin.
I'm referring to your doubt of a tied value peg between the two units
I don't doubt that it is possible to peg it, locking the bitcoin and then guarantee the conversion back to bitcoin at the pegged rate. Only that the economics of it doesn't work as the authors believe. Since it has different characteristics, (that is the point) (and don't forget the difference in liquidity), it would have a different value if it was floated. Therefore, the peg with no restrictions in conversion volume will necessary make value either move fully to bitcoin, or fully to the sidechain.