Why can't chocolate be made caffeine free?

When taken literally, the article you cite is simply wrong. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5302282/ -that's scientific proof that decaffeinated cocoa does exist. I have no idea if someone is actually making chocolate bars out of it, but there seems to be no theoretical hurdle for it. There is some remote possibility that the decaffeination may change the physical qualities of the cocoa powder, or that it is not well suited to other cocoa intermediate products such as cocoa butter, which could make producing a decaffeintated chocolate bar challenging - but even if that's the case, I would expect professional food technologists to be able to work around that.

I won't speculate on why the site supplied incorrect information. My personal opinion is that I don't trust sites that feel comfortable making broad health claims, and the site you cite is very much focused on doing exactly that.

In theory the process of extracting caffeine (as in coffee beans) can be applied to cocoa as well. I think the amount of caffeine is so low with regards to the consumed amounts, so decaf chocolate is probably not economically viable.