It is a very interesting question. My observations:
1. Before the end of World War II, citizenship in most countries was not a well defined aspect of law (though it was in some countries).
2. In many monarchies, subjecthood was well defined in law, but citizenship was not.
3. Before the end of World War II, statelessness was not well defined, nor did it have the same sort of impact it does today.
All things considered, I would not consider this person to have been "stateless", but rather, to have lacked the protection of any state at that time. On the other hand, if they were filling out a Canadian citizenship application today, I would recommend that they list their status as "stateless" during the time they lacked any state's protection, or as the citizenship they had when they first emigrated from their country. (It's a difficult call to make.)
Addendum: Before World War I, many people could not accurately answer the question, "What country are you a citizen of?" Many people would not even have known what "country" they live in. What they would have known is that they pay taxes to the taxman, and subjugated peoples would know their region was occupied, though not necessarily by whom.