One more vote to change water pump along since most of the work is already done for disassembly of the belt and tensioner but it will cost you some coolant as well. Its easy work though if belts are removed already.
Yep the part number of this OEM belt tensioner is really confusing. And you received left-over stock PET2-15-980 as I did go to my Mazda dealer and checked the actual situation in Mazda’s parts system. The PET2-15-980 has been superseded by PE03-15-980C months ago, and PE03-15-980C tensioner is the one they’re getting currently from Mazda. But when I compared 2 belt tensioners side-by-side physically, strangely the serial number on newer PE03-15-980C tensioner has older number F-569897.10!