I think that what he really means is that the best way to ensure the predominance of Conservative ideology is to ensure that the main opposition party broadly accepts everything that the Conservatives say. Conversely, an opposition party which says something different creates a real danger that people might start to understand that there really is an alternative to the Labour-Tory consensus of recent decades.
Cameron was a bit more honest, when he said about questions such as nationalisation and Trident “These are arguments I thought we had dealt with…” – i.e., that Labour has long accepted the Tory position. It’s easy to see why he would prefer to have both the two main potential parties of government broadly in agreement on most issues. It helps to legitimise the prevailing ideology, and ensure that people have no credible electoral option to vote for anything other than minor change.
Anything that challenges any part of that consensus, rather than simply falling into it to be seen as being ‘electable’, represents a move away from the two-faction one-party state which the UK has effectively become. It remains to be seen, though, whether that move will be as large or as sustained as its supporters claim. I’m still sceptical.