You need to run it as root to get accurate reports from cpupower;
boost states mean mostly will clock down individual cores to 0 (run turbostat to see this); and 400mhz is a low power pstate ; part of the way zen3/4 work is that using CPPC hints per core the algorithm internally calculates what limits seem safe/attainable and dynamically changes the reported pstates 400mhz . There is a concept of power/perf ratings which is the metric number you see in the middle of the Pstate reporting in the output attached.
If you want to understand how the amd-pstate module works the kernel documentation is your goto for detailed info.
