That distinction doesn't really exist. You have to pay employees. You have to pay for equipment. You have to pay for office space. And the countless other costs of running a company. There's no way they can realistically say "this particular dollar we got from you was used for exactly this purpose".
A budget that is influenced by the budgets of everything else. If you have $100 in free 'foo' money for 'foo' efforts, suddenly you do not need to fund 'foo' from your general fund and you can move those dollars you would have otherwise used for it to instead be used for anything else you want.
Their FAQ is not really clear on that, my feeling looking at this page is that it is more like: this money goes directly in the pocket of GOG, so by helping us to generate more money for us, you are helping (indirectly) the preservation of games as it is what they do.
But I saw nowhere in the FAQ explicitly that this subscription money will only be used to the "active act" of preserving other games.