Is EU Single Market membership part of the backstop in the Withdrawal Agreement?
No, it's not. A customs union doesn't equal the Single Market. Not even the Customs Union equals that. Turkey is such an example. Crucially, the Single Market includes freedom of movement for labor, which is not included in the Customs Union.
Under the backstop Northern Ireland will basically apply the full code of the Customs Union, while the rest of the UK will be in a less encompassing one.