The interesting bit with the DPA is that the DPA scheme itself probably doesn't have enough funds to cover one of the large 4 banks if they default. They probably have enough for a mid-size bank only. In the event of multiple bank defaults at the same time, their funds would definitely be inadequate.

Of the 47 basis points they collect on deposits these days, only one basis point actually goes into the fund that the DPA would pay out of. The rest going into paying off previous debts.

The government  or related ministry would probably step in though, and I'd say reasonably safe in THB terms if they did go under.

For me the Moody's ratings are just another factor to consider in the whole picture, and as a general rule a better rating means they would less likely hit a credit event, whether that be a bond default or other credit default, so less likely to need the DPA to kick in and less likely for the government to need to bail out the DPA if insufficient. They're all just probabilities.

I'd say no bank in the world is 100% safe though, even with deposit guarantees and government guarantees etc.