It is not so now. The policy has obviously changed. I know of several vets who received VA compensation settlements >30% (Special Disabled) for pre-existing conditions which were deemed to have been made worse by military service.
I met a retiring First Sergeant a few years ago who even though he had no disabling injuries and was not medically boarded by the Army was still planning to file a claim with the VA for 'the stuff that got worse' during his service. It was part of his retirement planning. I know of another Army vet who was medically seperatated for a pre-existing knee condition. He eventually received a 100% VA disability for his pre-existing condition. In fact under the photo on the VA ID card it lists if the injury is Service Connected or not.
Are you aware that every penny a retiree receives from the V.A., an exact amount is deducted from his military retirement check?