"New engine" is a marketing term. Game engines are a very generic and ill-defined concept. You write a game, and then you refactor a lot of the code so it can be used in a later game. This alone is often enough for the developers/publishers/marketing department to say that both games run on a modified version of the same engine, but that's no more or less valid a claim than saying because you made so many changes it's a new engine.
What they have done is rewritten a lot of the code they had. They have not thrown away any of the code that worked and did what they wanted, because that would have been stupid.
If a lot of the previous game code concerning stuff like rendering (this in particular), input/output, physics behaviour, etc. is brought back, especially if it's certain quirks characteristic to that engine in particular, then it's pretty safe to say it's the same engine. I don't think they brought back some of the same bugs and problems out of sentiment, or because that did what they wanted.
You can make something on the same but modified engine that differs a lot from the original (such as The Witcher 1 compared to Neverwinter Nights' Aurora engine), but there's nearly always certain tell-tale signs of it being the same engine (such as Geralt's inability to get over shin-high obstacles). Once you actually flaunt "new engine" as a feature, then it better be like at least twice as suitable to your needs like The Witcher 2's engine was (which by the way, bears no real resemblence to the Aurora engine and runs like ten times better).