IMHO, it is the subjectivity, as well as ignorance and, sometimes, an intentional desire for some to be misleading.
I agree with this statement regarding tube grading. But there are many more variables than record grading. Matrix numbers or artwork can be used to ID a record. Tubes have factory codes indicating date, batch, country. But NOS tubes also can be rebranded either by a parent company, or deceitfully by an unscrupulous distributor. Usually there are the factory codes to prove the provenance of the tube, but if the codes are missing it takes an expert dealer to identify the tube by examining the internal structure.
Tubes are different than records in that they can be measured by test equipment. But like a record, there’s no sure way to know if the tube is new as in never used, or nearly new. These tubes may have test results that are in range of a new tube even though it was used and put up for sale as new.