Possibly, but I always thought the rule is that if you can take the words between the commas out and the sentence still makes sense, then commas are correct. It's also apparently correct to use them before words like who and which.
Also the word 'which' should be used to separate a non-essential clause from the other part of the sentence. So the sentence 'Harry was driving his car, which he bought from his friend' is the correct use of 'which'.
In your sentence, the two clauses about punctuation and powers of expression are clearly linked, meaning 'that" is the proper usage.
It should read '
Funnily enough, I was thinking just the other day that I wasn't clear about the difference between 'that' and 'which'. You've helped me clarify that, so thanks.