What is important to note is that LiPo cell IR is a derived value. It is not a direct measurement or resistance, rather it is determined by measuring voltage drop below open load voltage and a known applied external resistance. This makes it very useful as it demonstrates the ability of a cell to deliver power under load, a direct measure of efficiency. And yes, as you say- it is a very useful indicator of cell health. Reduction in capacity is also a useful indicator as is cell balance across a pack. The DJI smart board is able to correctly report actual capacity, it performs IR monitoring of individual cells also during charge and discharge (amongst other things).
Not wanting to start disagreement - but a batterys overall capacity never changes even when dud ... its USEFUL capacity does though.
A mistake some users makes is to measure IR with for example a LiPo charger and then later compare to results via an IR meter OR at different state. IR is the best indicator of cells degrading - but needs to be compared using same apparatus, at same state and temperature.
Anyway - DJI do not allow us to access to measure it properly.
Nigel