Flew with them on Tuesday and was told by the very helpful manager of the swampy desk that you need either PCR or ATK test to transit through Singapore, regardless of final destination requirements
I was refused a flight when I had a one way ticket to Thailand. I did as others have suggested, booked a cheap flight out of Thailand on my phone and then showed this to the airline.
Approved test and go passes now have no cut off date. You can bring flights forward if you email them. If you follow Richard Barrow on Twitter you will see all of the latest details
https://twitter.com/RichardBarrow?t=X94ojvM9kcmEE7zQEGhrkA&s=09