Problem is that evidence gathering techniques can usually be reverse engineered by releasing the evidence gathered. If CSIS keeps an eye on foreign diplomats (a fair portion of the people working at any embassy are foreign intelligence), you don't want them to know specifically how we're gathering information on their intelligence actions in the country. Especially on something ongoing like the Indian efforts to interfere in protected speech in Canada.
So yeah, I feel you on that, politicians are liars and rarely deserve the benefit of the doubt but we also can't go giving away intel gathering methodology. Sometimes the government has to act on what it knows and can't/shouldn't make public.