Right from the Government of Canada's website: "An Educational credential assessment (ECA) is used to verify that your foreign degree, diploma, or certificate (or other proof of your credential) is valid and equal to a Canadian one."
A pilot's license is just that, a license. It is not a degree, diploma, or certificate. That's why Transport Canada said they can help with the conversion - you convert your license, you don't assess it. Unfortunately, the studies towards an ATPL outside of a university or college are somewhat treated like a driver's license. You don't consider the theoretical driver's test to be academic in nature and so too with the ATPL exams. Now, if you had gone to University and obtained a Bachelors's degree in Science and a pilot's license came as part of that program, then you'd use the ECA for the degree, but not the license.
Your best bet is not to talk to the internet, but to go find an immigration lawyer or consultancy service. They'll be far more up-to-date on the current regulations and requirements than anyone who has gone through the process and may be able to suggest a different immigration path for you.
I'd also take a hard look at the prospects for pilots in Canada right now and probably as far out as 2024-2025. There have been mass-lay-offs with more to come at most of our airlines and almost everyone has had to cave into contract concessions to help keep the airlines afloat. It's going to be a long time before the airlines are able to go back to full pay and benefits and even longer before they hire pilots as all those who were laid off to have to be offered their jobs back first. I'm not saying don't come, because your reasons for doing so are admirable. Just know that it's going to be a long, hard road.