The wording is awkward, but I think you are referring to this:
- Nonimmigrants who departed the United States for brief travel to Canada, Mexico, or an adjacent island (for F and J nonimmigrants) for thirty days or less;
The F and J portion refers to the adjacent island. So nonimmigrants generally can go to Canada and Mexico and come back using this rule. F and J holders can additionally go to adjacent islands (i.e Carribeans). Note: this was the wording when I looked up the rule many years back. This is an explanation of the rule from Dartmouth's international office:
https://ovis-intl.dartmouth.edu/immigration/h-1b-employees/h-1b-travel
AUTOMATIC REVALIDATION
After initial entry to the U.S., nonimmigrants who have an expired visa in their passport may re-enter the U.S. without obtaining a new visa if their travel was solely to the contiguous countries of Canada and Mexico.
F and J visa holders may also be able to benefit from Automatic Revalidation after travel to the Caribbean Islands (excluding Cuba). Travel must be for a period of fewer than 30 days. This procedure is known as Automatic Revalidation of the U.S. visa.
And it looks like based on some anecdotal evidence, this rule still works (based on a few reddit threads)
https://www.reddit.com/r/h1b/comments/q2l1fk
https://www.reddit.com/r/askimmigration/comments/e5v8gy
For me, I did not want to spend more time applying for a visa. CIC for whatever reason took a bit longer than what was expected, so by the time they issued my PR visa I only had 1.5 months to land (due to medicals close to expiring). The worst-case scenario was that automatic revalidation didn't work, and I would apply for a visa while in Canada as a Third Country National, but it didn't come down to that.
EDIT: Here's the relevant regulation...
https://www.ecfr.gov/current/title-22/chapter-I/subchapter-E/part-41/subpart-K/section-41.112#p-41.112(d)(1)
(d)
Automatic extension of validity at ports of entry.
(1) Provided that the requirements set out in
paragraph (d)(2) of this section are fully met, the following provisions apply to nonimmigrant aliens seeking readmission at ports of entry:
(i) The validity of an expired nonimmigrant visa issued under INA 101(a)(15) may be considered to be automatically extended to the date of application for readmission; and
(ii) In cases where the original nonimmigrant classification of an alien has been changed by DHS to another nonimmigrant classification, the validity of an expired or unexpired nonimmigrant visa may be considered to be automatically extended to the date of application for readmission, and the visa may be converted as necessary to that changed classification.
(2) The provisions in
paragraph (d)(1) of this section are applicable only in the case of a nonimmigrant alien who:
(ii) Is applying for readmission after an absence not exceeding 30 days solely in contiguous territory,
or, in the case of a student or exchange visitor or accompanying spouse or child meeting the stipulations of paragraph (d)(2)(i) of this section, after an absence not exceeding 30 days in contiguous territory or adjacent islands other than Cuba;
