My argument rests on the claim that “Member of Parliament” isn’t just a descriptor, but a specific term – essentially a title – that specifically denotes an elected member of the House of Commons. As per Wikipedia:
A member of parliament (MP) is the representative of the people who live in their constituency. In many countries with bicameral parliaments, this category includes specifically members of the lower house, as upper houses often have a different title.
The above is actually rather poorly worded; it’s apparent that by “includes” they meant to say “refers only to”. I don’t think you’ll find many people in Canada who would consider the Queen, the GG, or Senators to be MPs, even though they all have parliamentary roles.
Whereas “member of Congress” is, in fact, just a descriptor, in my view.