Resolved! hsi_ck should be hsi_ker_ck in the reference manual (0481)
In RM 0481 (STM32H563/H573) Rev 1, on page 444, Figure 48. Clock tree, HSI RC 64 MHz output is written as HSI (hsi_ck), whereas LSI and HSI48 outputs are named as lsi_ker_ck and hsi48_ker_ck. Since there is also a HSIDIV, and it is documented correct...