Their success is not directly from SEO but SEO amplified their word-of-mouth. They have also acquired brands that has helped but never ever forget - a killer solution to a killing pain point is what sets you up for success!
I know of a company with a team of 10 just managing Google shopping ads. Their entire SEO/SEM team is probably 60+. And they are only doing a million a day in sales. At Canva's size I think 75 is small.