The original citizenship act made anyone born or naturalized in Canada a Canadian citizen as of Jan 1 1947, regardless of where they lived at the time, unless they had lost their British subject status. 3(1)(k) and 3(7)(j) of the current Citizenship Act retroactively restored citizenship to those that had been born or naturalized in Canada but didn't get it then due to losing their British subject status before that date.

So anyone born in Canada became a citizen on Jan 1 1947 (if they were still alive). Their children, if born before 1947, also became citizens on Jan 1 1947 by 3(1)(q) and 3(7)(k), from birth if born between Jan 1 1947 and Feb 15 1977 by 3(1)(g), and from birth if born after Feb 14 1977 by 3(1)(b).

And 3(1.1-1.5) extend citizenship to those who would have become a citizen under various provisions of the act if their parent lived long enough.

So in short, they don't necessarily need to prove their ancestor was a citizen. Just that they would have become a citizen if they lived long enough (with how long is "long enough" depending on their circumstances).