If she has set the UK the target of leaving the EU, and replacing everything necessary to make that a smooth transition, undoing 40+ years of integration without wrecking the way the UK's economy and trade functions, etc, etc... then yeah, that's impossible. I find it hard to believe that Theresa May doesn't know this, but she probably feels (with some justification, perhaps) that it's better to press on with implementing the referendum result as far as she is able regardless of those difficulties.
To put it bluntly, the only people deluded enough to think that this was remotely achievable have all abdicated responsibility for implementing it, so can get away with saying anything they like and never be held accountable for their lies.