There is DRAM which is mildly defective but got past QC.

There are power suppliers that are mildly defective but got past QC.

There are server designs where the memory is exposed to EMI and voltage differences that push it to violate ever more slightly that push it past QC.

Hardware isn't "good" or "bad", almost all chips produced probably have undetected mild defects.

There are a ton of causes for bitflips other than cosmic rays.

For instance, that specific google paper you cited found a 3x increase in bitflips as datacenter temperature increased! How confident are you the average Firefox user's computer is as temperature-controlled as a google DC?

It also found significantly higher rates as RAM ages! There are a ton of physical properties that can cause this, especially when running 24/7 at high temperatures.