More of "we make this condensed core, based on Scala - and if we are lucky and it turns out that the current Scala core can be made sound easily, then it will become Scala 3, else it will be a completely separate language".

E.g. if it would have turned out that you can't make implicits sound, then I doubt that dotty would have become Scala 3. With existential types however, they decided to remove this feature (which is a bit sad imho) and sacrifice it to make dotty become Scala 3.