My interpretation is that it's not a legal requirement. It's political pressure that would prevent Canada from signing.
On November 1st, 2017. See this statement in context.
On November 1st, 2017. See this statement in context.
November 1st, 2017 / 4:05 p.m.
Professor, Royal Military College of Canada, Department of Defence Studies, As an Individual
My interpretation is that it's not a legal requirement. It's political pressure that would prevent Canada from signing.
See context to find out what was said next.