They already offer download servers for free, and I doubt a credit card fee comes close to 30%. I'm not positive what their reasons for choosing to charge on the app store are, but "they couldn't self host it for cheaper" isn't it.
There's a bunch of other related costs associated with commerce on apps. This is why some developers don't "charge" for apps or don't setup their own storefronts. Take credit card charge-backs or fraud disputes. By going through the Apple Storefront, Document Foundation is outsourcing that headache to Apple.
You know what also solves that headache? Offering a cost free download of your software. Like LibreOffice already does for MacOS, which they say is the recommended source for all users.