Well, he's on a bit of a bandbox, beating his chest for rightousness, no?
It's been found that anywhere from 55% to 80% of horses bleed after a race - worldwide. All countries. Even the ones that don't race on lasix.
It's known that those with more severe bleeding don't place as highly (win) as those with less severe bleeding.
Horses that bleed can easily have permanent damage to their lungs (related to severity and frequency).
If a horse bleeds, it can't race in other parts of the world (= wastage). If it bleeds in the US, it can very often continue to race.
Lasix doesn't mask anything, really, regarding other drugs. Detection nowadays is far to sophisticated to be influenced by a little dilution in the urine.
Exercise-induced pulmonary hemorrage (EIPH) is thought nowadays to have multiple contributory factors, high intrapulmonary pressure being only one (lasix is supposed to attenuate the exercise-induced rise in pulmonary capillary pressures that contribute to bleeding into the lungs).
You might google "EIPH" and "Lasix", and read stuff from the AVMA, AAEP, on The Horse website, etc.
I don't think the use of lasix, or not, is a simple black and white issue.