I think Boaz Yaniv's answer in the question you lined explains the meaning of 全然わかる very well.
The situations I've seen the positive 全然 are all situations where there's an expectation of something "not being X" but it actually "is X". For instance, you'd usually say 全然OKです when someone asks you if something is okay with you and you see that he's not sure whether it is - so you assure him that his negative expectations are completely unfounded and it's completely okay.
全然わかる can be used, for example, after you said you are worried about your Japanese skill:
You: (I think my Japanese is not good...)
... or after someone said you appear not to understand Japanese:
In such a situation, 全然 in 全然わかる implies "quite contrary to someone's current (negative) expectation / observation / worry".
Note that 全然 does not imply your current level of Japanese, at least directly. 全然わかる does not necessarily mean your Japanese is perfect. Basically it sounds like "Your/Her Japanese is not bad at all." or "She does understand Japanese."
This type of 全然 is still slangy, so please don't use it in formal settings.