Pretty much everywhere remains open, there is not the 'concept' of closing up for a low season, there might though be a few cases where perhaps people who are from other provinces 'might' close up a place perhaps July-Sept but that would be fairly rare, but you needn't expect a 'ghost island'.
May is only the start of the rainy time, you could be lucky and have solid sunshine, you could get a few short rainfalls, could be longer, but May should not be that much of a risk.
During the middle months though you could get a rough ride getting there or back, but it's not that far.
You should be fine in May but of course not guarenteed, days that it rains vries year to year, how much, how long.

My inlaws visited Ko Lanta in May last year and stayed at the Pimalei. They loved it. However the ocean was rough and they weren't able to take the speedboat transfer to the beach in front of the hotel.

We have also done the Pimalei and would go back in a heartbeat.