They aren't/weren't all going to be EVs. Originally the USPS was looking at 10% EVs and 90% internal combustion but there were a bunch of challenges to that (including that ratio being based on incorrect environmental impact data). Now it's 75% EVs in the first order of 60,000 and future orders may be that high or higher (currently expected that it will be all EVs starting in 2026).

sources: article and wikipedia (https://en.wikipedia.org/wiki/Oshkosh_NGDV)