I have just now tried at least to get there using TWRP’s file manager but it doesn’t look like it would even get you to the top level folder or it may just not show all folders.
After looking through once more I remembered that the system partition specifically has to be mounted. Going to TWRP -> Mount -> System (keep Mount system partition read-only unchecked of course).
Now there is full access and the bootanimation.zip can be replaced using the preferred way by the filemanager or command line.
It worked and looks nice.
IIrc I did not had TWRP installed yet back at that time when I followed this bootanim thread. So I thought rooting was the only way which I did not wanted to go just for this purpose. So I put it aside again.