I actually agree with your sentiments that the Caribbean is a bad choice for most students, but the 67% match rate is based off of first-choice match data, meaning that 67% of SGU students get their first-choice match. It doesn't mean that the remaining 33% don't match at all. Another non-trivial percentage get their 2nd or 3rd choice residency match (we know this data ahead of time, so the vast majority of students pick a backup residency), a few more percent get residencies outside of the Match, and a few never pursue their US applications because they get a Canadian spot via CaRMS. The exact actual residency match rate isn't known to my knowledge, but estimates put it at closer to 85-90% of students.
As you mention, however, that doesn't incorporate the attrition rates in MS1 or MS2, and it doesn't tell you anything about the quality of the program into which the applicants match. I agree also that data indicate the match stats are trending downward for Caribbean students.
Step 1 scores aren't the end-all, be-all of applications. However, a 260-scoring SGU applicant with commensurate Step 2 scores and LoRs, should almost certainly be able to get a residency somewhere in something, assuming they're not a blithering idiot otherwise. That being said, if you're already a student from UWI, chances are good that you will not be able to transfer to a US-based curriculum school like SGU without at least repeating 3rd year. So I'm not sure what your exact question is. UWI is a good school, but it caters to doctors that want to stay in the Caribbean to practice. Schools like SGU and Ross cater to US-based medical education. A US-based residency program is much more likely to have heard of SGU rather than UWI. If they're unaware of UWI's curriculum, there is a very good chance that they will see that you're from an unknown Caribbean school and dismiss your application outright. I think there is an argument to be made that a 240 Step 1 applicant from SGU would have an easier time getting a residency than a 260 Step 1 applicant from UWI.