In the absence of any other evidence, I'd assume the author of the footnotes to be the same as the author of the text, or the preface/introduction (or the translator). If there is evidence against that (e.g., earlier editions published during the author's lifetime lacking the footnotes), I'd assume the footnotes are "anonymous", subject to default publication date + 70 years copyright.
This is of course no legal opinion, just the rules of thumb I'd apply if I find such a work in our library
