Vaccination rates in immigrant populations is lower than native born children, but not significantly. As you noted in your link, they're generally willing to get vaccines, but many didn't have access before they immigrated.

Also, the current outbreak was incubated in Mennonite communities before spreading wider. Despite living in Canada for over 100 years Mennonites are still considered immigrants by many because they are insular and different.

In other words, it's a racist talking point that has enough truth to make it non-trivial to challenge, but has no actual basis.

Canada has a mandatory immigrant medical exam for anybody applying for permanent residence or a visa of longer than 6 months. At this exam, the doctor will ask for vaccination records and prescribe missing vaccinations. It is unlikely that immigrants are contributing to this outbreak.

The Mennonites in Canada aren't actual immigrants and are not subject to this mandatory exam.