Yes, the international pool slots are only of value if they're used. But the Phils wouldn't have made this deal unless they intended to use the slots. Does anybody disagree with that? I just don't see any other explanation for the trade.
If they just wanted to get rid of Eshelman (the AF argument), and didn't have an immediate need for international pool space, then the rational trade, with the Orioles or somebody else, would have been for another minor-leaguer...and clubs can always make such deals. That they traded for pool slots, and not for some AAA filler (they could use an outfielder at AAA, for example), tells me that they wanted the pool slots - and that implies that they have a specific use in mind, at this late date in the signing period.
Eshelman will be eligible for minor-league free agency after the 2021 season. He was Rule 5 eligible last December, and not protected (and not drafted). That was before his month-long implosion this spring, so I don't think his perceived value has increased. So I agree, there's no way he was going to be added to the 40-man this winter - his only path to the Phils' 40-man roster would have been if he pitched lights-out in AAA, and still more other starting options for the Phillies went down.