I think you are right. I honestly think he and his campaign got into this thinking Jeb would tie up the nomination and he would build a profile to get elected governor of a major state. Then maybe run for President down the line.
Instead the non-establishment candidates totally took control, Jeb imploded, and the establishment turned to him as the highest polling establishment candidate despite him having almost no campaign infrastructure and little money.
This is going to be like the UK election where the polls were mostly wrong. I'm willing to bet a substantial amount there's a lot of shy Trump voters out there. A lot.