Which open source license are they released under?
https://creativecommons.org/licenses/by/4.0/
Other premium icon sets aren't open-source, so using them would require a backlink or a purchase.
CC BY is recognized by the FSF as a free license and by Debian as DFSG-free. While it's not listed by the OSI as open-source, I think that's mostly because it's not a code license. So I don't think that calling it a open source license is misleading.
Of course, for open source icons I would actually expect the source files to be open...