Can a Canadian (non-U.S. person) buy US T-Bills from TreasuryDirect.gov?

No. Foreign investors can buy T-bills, but not through treasurydirect.gov.

From the User Guide:

In the Submission section you certify:

1. You have provided your correct taxpayer identification number. (This is typically your Social Security Number.)
2. You are NOT subject to backup withholding.
3. You are a U.S. person. (You must be a U.S. citizen, resident, or individual who is at least 18 years old with a valid U.S. Social Security Number. You must also have a U.S. address of record and an account at a U.S. depository financial institution that will accept debits and credits using the Automated Clearing House method of payment.)