govt has no need to employ more officers than they need for the mega draw. they'd have invited the same number over 3 months. they did it in a single draw because they need people to apply asap. cec was a good call for that because they are quicker&easier to process => less cost per application. also, they couldn't justify inviting 27k outlanders with a crs cutoff of 200 to anyone, so they did cec.
this has nothing to do with the govt's promise. remember that people with crs300 maybe less got invited. the intention is not to invite literally anyone who worked in canada for a year, but to invite people with high qualifications. you get more points and a dedicated stream if you have contributed to canada's economy by paying taxes at least for a year. however, you still need to possess other qualities such as good command of english/french, education etc. crs300 or even less who got invited doesn't fit this profile, but it's the pandemic.