Had a little bet with a distributor. Full-on dinner at Tony Roma's drinks and dessert. His argument was the same as yours. RJ's fishing. "They all have cheesegrater testers".
His incidence of testing OK but having the battery come back bad in 90 days was >30%. Mine was zero 0.0000000%
Auto Meter's 800 ampere tester sold for in excess of FIVE HUNDRED DOLLARS. And Other than Sun's VAT 33 and VAT 40, it was just about the only full BCI loading method of testing batteries correctly. THAT's WHY THEY USE THE CHEESE GRATER LOAD TESTERS. "Better-then-nothing, is not the same as ADEQUATE".
3 VO & Water, King Size order of baby back ribs and coconut chocolate pie coming up!
If you contact the BCI and ask them about the relevance of testing batteries with a cheese grater, you will hear silence, then a chuckle.
This is as silly as saying one of those "Five Floating Balls" hydrometer thingees can substitute for a Francis Freas hydrometer, or a Stewart Warner dash gauge voltmeter will suffice for battery monitoring. Yeah 28 dollars when I purchased 30 of them. Ooooo what a surprise. 12.00 volt power supply, readings from 11.5 to 13 volts. The official reply from Stewart Warner? "Well, they're meant to give an acceptably approximate reading".
Chesse graters. Better than nothing, but not by much.