Translation:Nobody comprehends the stock market like he does.
'like he does.' is not strictly correct English. It should be 'as he does.' The rule is that 'like' is used with a noun, 'like my brother' and 'as' is used with a clause, 'as he does'. I know a lot of English speakers use 'like' instead of 'as' and maybe this usage is becoming the normal, but I expect Duolingo to stick to English grammar as it is currently, just as I expect it to teach correct German grammar.
I have the same nodule in my memory, but current usage now accepts "like" as a conjunction. See these links for US and British usage, which seem to endorse Duolingo's usage in the English sentence: http://www.merriam-webster.com/dictionary/like http://oxforddictionaries.com/definition/english/like?q=like
The point in any language should be to be able to communicate your thoughts and understand those who speak to you. "Like he does" is used often in spoken American English, and while I agree that duolingo should always accept the correct grammar translation, I appreciate that it takes into account normal spoken English expressions.
There is an ambiguity in both "like he does" and "as he does" namely that it could mean "Nobody understands the stock market as well as he does" or "Nobody understands the stock market in the same way that he does".
Is that ambiguity present in the German sentence?
Is the translation "Nobody understands the stock market like he" correct English?
Your translation definitely 'catches the gist' and you will often hear "like him" in spoken English. However, Duo may have marked it wrong because it is technically not grammatically correct to say "like him". It should be "like he does" or "as he does". See the remarks above.
Duo's currently accepted translation "Nobody comprehends the stock market like he does" suffers from bathos, i.e. there is a clash of style between the somewhat elevated comprehends and the colloquial ...like he does. Bathos is sometimes deliberately used with humorous intent. Sentences that work as intended:
Nobody comprehends the stock market as he (does).
[elevated style throughout; does is optional]
Nobody understands the stock market like he does.
Though it's not an issue here, I recommend English learners to use the adverb like = as very sparingly to avoid any confusion with the verb to like in some contexts.
[2 Mar 2019 09:16 UTC]