I accept that campaign finance laws may be hard to understand. I accept that they may require interpretation. What's funny, though, is that the Cons' lawyer isn't saying: "Okay, look, maybe we did something wrong, we're not sure" or even "That's not our interpretation of the law, but we'll puruse it in court". Instead, we get a superlative claim that there has "never" been such an interpretation of the finance law. Given that this lawyer has probably not read the case law, I find that hard to believe.
Moreover, I find it hard to believe that the Cons would have let the Libs get away with that sort of a response.
Moreover still, it strikes me as pretty evident moral weakness. The lawyer is not claiming that the Cons didn't do it. The lawyer is not claiming that it isn't illegal. He's disputing whether the interpretation of the law can be stretched to cover this particular action. This is not a declaration of innocence, or that the law should be changed (perhaps to sharpen the interpretation), or anything of that nature. If the Cons didn't do anything wrong, why don't they just say so?
The answer that seems most plausible is that they did do something wrong, they know they did something wrong, and they're frantically trying to control the fallout instead of standing up and admitting it.