While methods using temperature and stomach contents are used for initial estimates, forensic pathologists actually look at the contents of various cells. The process of decomp is well studied and has several stages at the cellular level. Pathologists usually use several methods and the goal is that they all point to about the same time.
In this case, I believe at least one of the victims wore an Apple watch (E), so that could also provide a strong clue.
In the below article, the authors try to list the various methods in use in 2022:
The identification of a reliable and accurate post-mortem interval (PMI) is a major challenge in the field of forensic sciences and criminal investigation. Several laboratory techniques have recently been developed that offer a better contribution to the estimation of PMI, in addition to the...
www.mdpi.com
They use both blood and cerebro-spinal fluid and also the decay rate of various molecules in the body (that part is a lot like a clock, IMO).
It's a fascinating question and it seems all the various methods range from rough estimate to very accurate, with almost clock-like results. Of course, we don't know what the U of W pathologists did at Spokane, but I am confident that they have access to at least some of these recent and specific methods.