current source pull up
From what I understand, resistors are hard to implement in silicon
  and/or take up a lot of space, so instead something like a current
  mirror circuit is used, and the datasheet quotes a pull current.


Yes, this is correct, in general. Implementation of pull-up resistors is non-trivial using silicon technology. In modern MCU this function is usually configurable, and systematically is implemented as "I/O pad cell" in a universal form. This includes weak switching elements, which behave not exactly as resistors. Implementation of I/O pad cell libraries is highly proprietary and usually not disclosed to public, only in a form of IBIS models, or datasheets in a crude form of either as "effective pull-up current", or as resistor. In fact, the pulls are nonlinear elements, which are difficult to characterize by a single parameter. It is a choice of manufacturer to use whichever parameter suits the particular I/O better.

However, usually the pulls are weak relatively to the main functionality (drive ability) of the I/O cell, so in first approximation they can be modeled as resistors of corresponding ballpark value. Say, the pull-up is said as 58 uA typical. If the input LOW level is, say, 0.8 V, and Vdd is 3.3V, one can use 43 k resistor as a model, (exact definitions are difficult to find, as explained above). This shouldn't change overall logic functionality of a model. But if some design is marginal and sensitive to actual load values, a IBIS model should be used.