It's always good to have a "heads-up" from those concerned with safety, with liability.
All machines deteriorate with time. Some components or systems faster than others. For example, few HVAC systems in cars are in "outstanding" condition past six or seven years. Most are in need of component replacements.
Six years seems long enough, even for a collector car seeing 2500-miles annually. After all, what is the likely cost of repairing a vehicle after a blow-out, loss of steering/braking control, and accident? What is the cost to "repair" the occupants of this and other vehicles?
So, okay: 6-years or 4/32's remaining tread with no other damage affecting safety/performance. Seems little different than 6-mos/6000 on an oil change minimum (one I've used for many years on collector cars).
I might quibble with the recommendation, but prudence (and not being an industry engineer) leads me go along with this until or unless other other information succeeds it.
At the least, any vehicle with low annual miles deserves to be driven very, very gently for the first fifteen miles leaving storage after one has made a physical inspection of the tire exterior. Those tires need to heat up just as much as the fluids we're accustomed to thinking about. Probably a good idea to use an infrared heat gun to check for hot spots at the fifteen mile mark, and again when higher speeds have been run for, say, another fifteen miles.
Any other ideas to "determining" condition?