Is it true that resin fillings require negligible amount of tooth preparation when there is no decay in the tooth?
No Decay=No Filling. You are mostly correct; however, a big point to make is this: if you do not have decay, there is no need for a filling at all. Other than that, which may have just been a typo; yes, composite fillings do require less tooth removal, when compared to amalgam fillings. There used to be an old mantra "extension for prevention" when placing amalgam fillings. That is gone, and minimalistic is best!
Probably. Resin restorations require less tooth removal than do metal restorations, whether it is negligible or not. Other restorations require mechanical retention built into the preparation, whereas resins are chemically bonded to the tooth structure.