Tech stop doesn't need 5th Freedom. But having those handy would certainly add up revenue. We already have 5th freedom rights at BKK. But that is not the problem PAL would be having when it does fly the A330. Its enticing UK nationals to fly them to BKK that is the problem. The fare matrix there is already very competitive. A few BKK bound passengers would not make a difference.
In the same manner that PR flight to LAX via ICN does not make difference even if it takes 5th freedom traffic on the A330.
Making Sense