They would be allowed in as visitors on the IM-1? That's good to know; though I suppose they would have to either leave and re-enter, or flagpole to complete the landing.
https://www.canada.ca/content/dam/ircc/migration/ircc/english/resources/manuals/enf/enf04-eng.pdf
Section 12.13 on page 64.
12.13 Family members arriving before the principal applicant
Occasionally, a border services officer at Immigration Secondary will encounter a family
member who arrives before the principal applicant and is seeking permanent residence.
Paragraph R51(b) requires a permanent resident visa holder to establish that they and their
family members, whether accompanying or not, meet the requirements of the Act and
Regulations. For a family member to meet these requirements, it is usually incumbent on
the principal applicant being admissible at the POE. This also holds true for the principal
applicant arriving before their family members.
A border services officer encountering this situation should obtain the following information
from the family member or principal applicant:
why the family member or principal applicant is preceding the rest of the family
(e.g., to seek accommodation or employment, lack of a seat on the aircraft carrying
the principal applicant);
when the rest of the family is due to arrive; and
the person’s means of support.
The border services officer should complete the verification process but should not grant
permanent resident status to the family member. If the person has a valid permanent
resident visa and the border services officer is satisfied that the rest of the family intends to
come to Canada, the border services officer may wish to defer the examination pursuant to
section A23 in order to obtain more information or wait until the rest of the family arrives so
they may be examined.
The border services officer should enter the information into GCMS by means of a GCMS
Note, which indicates that the granting of permanent residence has been deferred pending
the arrival of the rest of the family.
If the border services officer has reasonable grounds to believe that the rest of the family
will not be coming to Canada, the border services officer should initiate enforcement action
unless the person qualifies in their own right for permanent resident status.