They weren't asking for a "single set of laws". "Single market" however is a pre-existing concept recognized as "one of the EU's greatest achievements" [1], and lacking regulations in providing said market is what their critique was about.
[1] https://single-market-economy.ec.europa.eu/single-market_en
> "one of the EU's greatest achievements"
And it is! People here are always complaining about the EU as if it's a single state, even though it's very clearly in the process of becoming one. It's a long road, we're not sure if we're going to make it all the way to the finish line (but I suspect we will), and we're going to have some happy little (and big) mistakes along the way. But for now it's imperfect, and it might be imperfect for a very long time to come. It's still a lot better than not having it, and we're actively working on improving it.