The point isn't whether or not skyler has him figured out. The point is that Walt can't help himself but lie. Perfect example is the coke machine you yourself referenced. He may not be good at it anymore but he's still damn sure going to try. And he's still keeping things from her like jesse & the fact he's making deals to still cook.
He's been nothing but calculative and a master of manipulation and lies the entire show leading up to him so desperately wanting to just flee with his family. And at this pinnacle point he has nothing.
Surely the 'calculative' Walt would have some sort of story worked up as to why they were going to up and flee before he hit the door.
Surely the 'quick witted' Walt would have adjusted the story on the fly (like he's shown to do all storyy arch long) to fit Skylar suddenly knowing about him being arrested by hank. Even if the story was fumbled and unbelievable.
And lastly the 'Heisenberg' persona which finally came through at the end wouldn't have just grabbed the baby and ran but would have attempted some sick and twisted lie/threat/manipulation before running with the baby.
Then afterwords Walt still shows he's still calculative, manipulative and a master of lies on the phone and returning the baby.
It just didn't fit.
It would have been much more believable if he had an outlandish story, then fumbled another lie on top of it... where skyler would then not believe him and call him out on it.