My mother is a permanent resident of Canada and a Chinese citizen. She booked a plane ticket to China. If she gets a visa on her Chinese passport from the Taiwanese consulate in Toronto, then flies to China, and visits Taiwan (for tourism purposes) before flying back to Canada, is going through Hong Kong International Airport virtually her only option in order to accomplish this? For some context, this visitor visa to Taiwan is open only to Chinese citizens with a work permit, study permit, long-term residence visa or permanent resident status in Hong Kong, Macau or a foreign country. It also appears that Chinese citizens are not allowed to take trips from China to Taiwan unless they are travelling as a tourist group as of August 1, 2019.
Well, technically, you have another option: travelling to Macao (or any third country)... But yes, her only option is to do so.
If you fly to Taiwan from the mainland, the Chinese immigration control will ask for a Taiwan Travel Permit issued by mainland Chinese authorities and a valid exit visa recorded on the permit. If you don't have both of them, you cannot travel to Taiwan from mainland China as a mainland citizen. However, no one really cares if you travel to Taiwan via a third location (that is to say, unless you are suspected of espionage or anything like that). If you travel via Hong Kong then you're fine.