He's a liar (well, that assumes he knows the truth -- he might just be mistaken). If the product were pure boric acid, then 5 pounds in 5000 gallons would give 21 ppm borates while if it were sodium tetraborate pentahydrate it would be 18 ppm and 20 Mule Team Borax (sodium tetraborate decahydrate) would be 14 ppm.
Perhaps you can show him the best case math with the product assumed to be 100% boric acid, B(OH)3, which has a molecular weight of 61.83 g/mole while Boron, B, which is how borates are measured has a molecular weight of 10.81 g/mole. 5 pounds is 2268 grams and 5000 gallons is 18927 liters so we have:
(2268 grams) * (1000 mg/g) * (10.81/61.83 boron to boric acid) / (18927 liters) = 21 mg/L (ppm)
The test strips aren't accurate enough for you to determine whether you've got 20 or 30 ppm borates -- that's too close -- but the math does not lie.