Details of Marlin's feedrate calculation
Feedrates are not 4-dimensional, and yes this makes them a bit inconsistent. But physically the 4-dimensional speed would not make any sense - for example, slowing down the E axis while speeding up the X axis would not maintain the same &quot;overall speed&quot; in any meaningful sense.
So, feedrates work differently for:

Moves with a nonzero X, Y, or Z component: the feedrate is an ideal, desired speed in 3 dimensions, possibly limited by the max feedrates of each axis (including E) individually, as well as their acceleration profiles.

Extruder-only moves where X, Y, and Z components are all zero: the feedrate is an ideal, desired speed in one dimension: the E axis, and may be limited by the max feedrate and acceleration profile for the E axis.