Interesting device! There's more details linked in the CNX Soft post [1], in particular, a thread on ODROID.com showing benchmarks [2]. The two disks are connected to the CPU via the ASM1061 SATA to PCIe x1 controller, meaning it can't fully handle max throughput from 2 SSDs, but I guess spinning drives should be fine.
[1]: https://www.cnx-software.com/2020/10/20/odroid-hc4-low-cost-...
[2]: https://forum.odroid.com/viewtopic.php?f=29&t=40609