I think the distinction is that MemoryMax= is just an interface to the cgroupv2 setting, i.e., that rule is implemented inside the kernel and invokes the kernel's OOM killer within a cgroup. The manpage for systemd-oomd says, "systemd-oomd is a system service that uses cgroups-v2 and pressure stall information (PSI) to monitor and take action on processes before an OOM occurs in kernel space."

It looks like systemd-oomd is related to (based on? from the same people as?) Facebook's oomd https://github.com/facebookincubator/oomd , whose documentation gives a bunch of reasons as to why you would prefer a userspace oomd that takes in PSI data and can be configured to proactively kill misbehaving processes instead of just letting the kernel OOM killer handle it. The major reason is time to recovery: a misbehaving process can cause a system to be so far under pressure that the kernel OOM killer will take a long time to flush things out, but a userspace component can respond in advance with more configurable rules (and more flexibility, since the kernel doesn't believe you're at capacity yet).