The existing definition has always been how I thought it was commonly understood. The basic idea was that it was wrong to recruit potential immigrants into a political party, either by discussion of party platforms or outright promotion. If the biggest political party decides that only the latter is wrong to do, then the norm is defined down accordingly. It's not like anyone can stop them. That's the thing about norms... they're not laws. A lot of people have thought it was important to keep political influence as far away from the immigration process as possible, but we have freedom of speech, so no one could ever make it illegal. If someone wants to chat with prospectives about a referendum on the monarchy or a Real Cosa possibility, then no one can stop them, really, even if that sort of thing starts to influence the immigration process.
It's actually kind of fascinating how fragile the stuff is. I mean, this conversation is probably basically it on the matter... the whole event. One politician suggests to another that they are coming close to violating a norm, the other politician replies that they have a different definition, and that's that. Sometimes in other circumstances, the politician will declare that they have no choice because everyone was going to do it anyway or that the other person's reaction is forcing them into it. In the United States, it's happened a lot in the Senate, particularly, over the last 40 years.