There is no such law in place.  The Safe Third Country Agreement designation of the USA is not predicated upon a claim having been submitted there - just coming from the USA. The eligibility criteria only cover prior refugee claims in Canada as set out in the IRPA eligibility criteria.

Once a refugee claimant is physically in Canada legally or illegally then under the IRPA implementing our international refugee protection obligations, the claim to refugee status must be assessed and the Charter of Rights and Freedoms applies. This was confirmed by the Supreme Court of Canada over three decades ago.- Singh v. Minister of Employment and Immigration, [1985] 1 S.C.R. 177.

The RPD will consider past actions in other countries as a matter of  credibility and the "well-founded" nature of the persecution subjectively and objectively.  The only disqualification would be a person who has been previously granted recognition and protection as a Convention refugee in a signatory country.