Another article, https://www.cbc.ca/news/politics/china-executes-canadians-drug-related-crime-1.7487764 explains that all four were in fact dual citizens.

However, from that article,

The embassy said the cases were handled “in strict accordance with the law” and that the rights and interests of the Canadians were “fully guaranteed.”

And from the original article,

China “protects the legitimate rights of the parties concerned as well as the consular rights of the Canadian side, in accordance with the law”

These together read to me like a coded reference to Article 12.3 of https://travel.gc.ca/assistance/emergency-info/consular/framework/china which states,

A national of the sending State entering the receiving State with valid travel documents of the sending State will, during the period for which his status has been accorded on a limited basis by visa or lawful visa-free entry, be considered as a national of the sending State by the appropriate authorities of the receiving State with a view to ensuring consular access and protection by the sending State.

I.e. these folks probably intended to just be treated like any other Canadian by entering China on their Canadian passports somehow. (One method might be to take advantage of the visa-free transit policy, see http://vancouver.china-consulate.gov.cn/eng/ggjry/202412/t20241219_11502288.htm )

I’m not convinced in any case that dual nationality had much effect on this, as China has enforced these laws before on folks who didn’t have it and clearly weren’t in any way Chinese, for example see this case of a British Citizen, https://www.theguardian.com/world/2009/dec/29/akmal-shaikh-execution-china

For the record, I’m against the death penalty and wish national conditions would emerge in such a way as to permit China to gradually abolish it. However, there’s also a lot of history behind why China is specifically so strongly against drugs (see https://www.abc.net.au/news/2018-09-02/modern-china-and-the-legacy-of-the-opium-wars/10172386 for an overview).