Honestly, this fits with the super-conservative nature of the island. Right now they say they don't even know how the community spread happened, and that has to be worrying for them. My guess is that someone broke quarantine and spread it. I'm not surprised.
I don't know what they expected - let's assume that their goal of 80% vaccinated was reached and they opened - they would instantly have cases - what would they have done? Shut right back down again? This feels like they weren't really prepared for what an opening without quarantine was going to look like, based on this.
I agree that they are going to need to come up with a plan of what their post-pandemic world is going to look like, because it is obviously here to stay in some form or another. Can they keep quarantine in place indefinitely and as a result completely shut down the tourism business on the island? Is that what their citizens want? Because if they aren't they are going to need to come up with a far better plan with a lot better contingencies built in.