The Alternative Vote lobby (including the IPPR) is arguing that the present First Past The Post system (FPTP) is based on the outmoded assumptions of 2-party politics, and also that the FPTP system is more likely to produce coalitions (like the present one).
These two propositions are not, quite, mutually incompatible. It is possible to want a voting system which better reflects multi-party politics yet which produces outcomes which eschew them. Possible, but ridiculous. Hilarious, even, when you consider that they’re asking Lib Dems to vote for it.
BTW, I really don’t care all that much either way. It’s chattering-classes twatterari. I haven’t had a single letter from a constituent about the matter.