I'm not aware that they "went on about winter fuel" as part of their election campaign. I don't think you'll be able to find a single reference to it pre election.
Now you can argue it should have been in the manifesto. But that's not the same as them "going on about it" then doing the opposite.
And for what it's worth, it was quite right to take it away from pensioners on very good final salary pensions who don't need it. They went wrong in not thinking through the consequences for those just above the pension credit threshold, just as they went wrong not thinking about the most vulnerable businesses and the NI raise, and not thinking through the farmers inheritance tax issues where farms are land rich but cash poor. But they are trying to tackle a huge deficit and there are definitely businesses and farmers who are not paying their way. So it is a complex set of problems to solve. the Tories also chucked people under the bus to save money (bedroom tax, two child limit) but they made life worse for the most vulnerable and marginal so there was not the same outcry.