The speeds officially supported by a CPU's memory controller tend to lag behind the speeds officially supported by the fastest available memory modules. This gap got pretty wide toward the end of the DDR4 era: Intel's current CPUs that support both DDR4 and DDR5 only officially support DDR4 speeds up to 3200MT/s, but there's little reason to equip a desktop with slower than 3600MT/s memory given the state of the DDR4 market.
6400 MT/s is similarly beyond the fastest officially supported DDR5 speeds for CPUs. But the fine print reveals that this machine is actually using soldered LPDDR5, which is officially supported at 6400MT/s or higher by pretty much every processor or SoC that uses LPDDR5(x). Ditching the DIMM slots really does make it easier to operate the memory bus at higher speeds.
Star Labs needs to correct their site to not refer to LPDDR5 as DDR5, because they really are different types of memory and only one of them is upgradable by the end user.