What does this answer about old residential home grounding mean?
Don't rely on MC as a ground

What he is saying is that it's not reliable to count on the metal jacket of MC (metal armored cable) as the ground.  

In other words, for grounding purposes, treat MC exactly like it is NM.  If there's a ground wire inside the MC, that is an acceptable grounding path.  Otherwise treat the cable and boxes relying on it as ungrounded.

Retrofit your heart out

Go ahead and retrofit grounds if you want to.  The post is saying since MC armor is not a valid grounding path, you will want to retrofit a ground in parallel.   Keep in mind the ground retrofit rules have eased since that post.  

When retrofitting grounds, think first about your biggest loads, because once you lay the thickest ground wire, it can also serve as a ground path for any other circuit(s) which require that size or smaller.  For instance when running your oven's ground, stop in a couple other junction boxes and use them as a hub for other grounds you are retrofitting.  That way you don't need to do a home run for every circuit.  The concept is that ground doesn't normally flow current, and it's rather unlikely for 2 circuits to develop a bolted ground fault at the same time.