Well there's no proof they lied, they simply decided not to support it when it came round to it. Unless they'd decided that beforehand, then rather than lying they changed their minds. And it's difficult to say they decided that beforehand as it was a different treaty then.
No proof they lied? From their 2005 Manifesto:
We are therefore clear in our support for the constitution, which we believe is in Britain’s interest – but ratification must be subject to a referendum of the British people.
When both the pro and anti EU people are happy to say that the Lisbon Treaty and EU constitution are effectively the same document then I would say that is a lie. Also when two Lib Dem Shadow Ministers quit and one gets sacked due to the decision to vote for a referendum I would say that the party went against their promise and knew they were doing so. So yes, the party lied on their manifesto.
Also, better they backtracked on something they said than going ahead with a stupid idea. I've no problem with politicians saying they were wrong about something and have changed their minds, reassuring to see they're open to reason.
Alternatively their manifesto promise was a sham as they knew that not promising a referendum when the other two parties did would be politically damaging. So I cannot believe what they say in their manifesto as it could equally be another political lie they will go back on.