Item #14 of their licensing terms reads as follows (they do reference the questionable “split” license):
14.For some items, a GNU General Public License (GPL) or another open source license applies. The open source license applies in the following ways:
(a) Some Items, even if entirely created by the author, may be partially subject to the open source license: a ‘split license’ applies. This means that the open source license applies to an extent that’s determined by the open source license terms and the nature of the Item, and this license applies to the rest of the Item.
Split and other open source licensing is relevant for themes and plug-ins for WordPress and other open source platforms. Where split licensing applies, this is noted in the Item’s download files: for more information, see this Help Center article
(b) For some Items, the author may have chosen to apply a GPL license to the entire Item. This means that the relevant GPL license will apply to the entire Item instead of this license.
Where an Item is entirely under a GPL license, it will be identified as a GPL item and the license noted in the download files: for more information see the FAQs