Doesn't Skyteam generally consider all flights within North America, including cross-border services between any of US/CA/MX, as lacking the "international" character required for Skyteam lounge access?

It certainly was like this up until the last few years if not still the case

EDIT: It seems it is now restricted to US-Caribbean being considered not international:



So the explanation above, that the lounge is only for AFKL pax, is the correct explanation.

Note that the KLM lounge at YYZ (Toronto Pearson) is open to a much wider slate of airlines, including Aeromexico: https://www.skyteam.com/en/lounges/l...on%20Intl(YYZ)
Aeromexico customers departing YYZ also have access to the Plaza Premium lounge there.