A determination whether you fall under any of the exemption category is for IRCC or CBSA to make. Even if you fall in one of these categories, you need to demonstrate it to the carrier and then to the CBSA officer at the PoE.
Currently, if you are not expressly exempt, then you may not be allowed to board. Also, as per the above exemption, it is only for the person who is providing an essential service, and not to his whole family.
If you are landing in Canada as a CORP holder then your COPR should have been issued before March 18, or you should be expressly exempt.
This would be how I interpret this. I know some people have claimed that they successfully landed and claimed essential worker, but that is all anecdotal and unless you expressly are exempt, you will need something to prove the same.