I have been looking into this site and I am convinced that "whom" should be used. If we take a look at the snippet of the quote in question, "[...] whom I appreciate [...]", I will place "whom" at the end and replace it with "them/they". It would now look like this:
"I appreciate them/they"
Which one should I use?
Clearly, as English speakers, we would choose "them" instead of "they" because we know that's grammatically correct. The article describes that if we use "them", or similar including "him" and "her", we should use "whom".