If you look at activity and what it's connected to via the whatId, it's represented as a lookup. It would seem that polymorphic is closer to MDR.
Any idea what's going on there?
Lookup is just a salesforce specific term for a foreign key relationship. Master-detail has a number of additional implications that generally are not true of polymorphic lookups.
For example, an object on the detail side of a MDR has no owner field, and is not able to participate in the sharing model; instead access to it is controller by the parent record. They're also able to use rollup summary fields and cross-object workflow.
While many polymorphic lookups cascade delete like MDRs do they don't always meet the full criteria for a MDR, and listing them as a lookup is more appropriate despite their special behavior.