If you look at the HN thread of the TWiST video, many were claiming they're just gassing the potential buyers up by saying they're not for sale.
I don't get why its really that hard to believe. Loving what you do is the thing I strive for, and they seem to have found it. Why mess with it?
People can say you can flip it and start anew, but 37signals has (according to DHH) enough to not worry financially and if they have a great new startup idea, why would they start a new company to do it? They have a company that they own and have the freedom to implement and launch products that they think are great.