I'm not a member of that forum, but you are actually trying to help the other person. I'm in favor of that. I'll offer this advice which I hope is somewhat constructive. The person wanted an explanation of what was going on. Here is my take on it and you will have to paraphrase.
When you open a file through Windows Explorer, there is a "default file" as part of the file dialog. For Explorer, that default is "*.*" (i.e. any file at all). Also, in Explorer there is no barrier to the use of the default "file associations" - the things that say that you use Adobe Reader on a .PDF and Access on an .MDB, etc. So opening the file from a folder goes through the "file associations." That means that a .PDF gets opened by an Adobe product.
That same file system dialog opened from a program such as Access or Excel might have different defaults for opening files. Like for Excel, the file selection string is perhaps "*.XLS, *.XLSX, *.CSV" or something similar. In that context, the file system dialog isn't going through Windows Explorer. Therefore, the associations might not apply in the same way because of the more restricted nature of the program doing the file-open step. That is, Excel isn't as powerful as Windows at opening files because it cannot apply associations that don't lead back to Excel.
I hope you can help that person. If this advice makes sense, well and good.
By the way, there are still times when you definitely are a jerk. But the thread you referenced wasn't one of them.