Mitch is pissing on your leg and telling you it is raining. He knows, most Americans are ignorant as to how the federal budgeting process works. If he was seriously concerned about the money actually going to the veterans he would have supported switching spending from "discretionary" to "mandatory".
See, much of federal spending is "mandatory" spending. The Social Security administration tops the list. But "mandatory" spending is also involved with things like the FDIC, unemployment compensation, and yes, Mitch's pension. And of course, the VA.
The bill made the spending on the treatment of Veterans suffering from medical conditions caused by the toxic fumes of burned waste MANDATORY. So yes, that is the only way to insure the money actually went to the veterans. If it were "discretionary", well then some yahoo future president could come along, proclaim an emergency, and confiscate that "discretionary" funding for other uses, like, say, building a stupid ass wall. I mean it is a fool me once fool me twice kind of thing.
McConnell either just don't want to give Democrats any victory, even if it harms veterans in the process. Or, he wants those funds to be classified as discretionary so he can tap them for some other purpose later on, after perhaps taking the Senate. Probably both. This excuse he is trotting out is insulting in its sheer audacity and extortion of public ignorance. But he shows, beyond a shadow of a doubt, that neither He, nor his party, give two shits about anything or anyone other than themselves and attaining power.