I don't know if you read it (because it's in French) but it basically says that it comes from shampoo (which comes from the Hindi champoo) but not why it comes from the gerund shampooing.
I googled for a good half hour and came across that link in my searches, but no clear consensus on why it stems from the gerund form.
The only thing I can think of is that the original Hindi word is "to lather/massage" so maybe the French took the English transliteration and merged it with its Hindi gerund origin to end up with shampooing?
According to the CNRTL link, in 1877 it was used in the same way we would use the gerund in English, to refer to the action ("signifying the washing of hair") then in 1890 became the word for the product used in that process (shampoo) as well.
That seems to happen from time to time in French, when the gerund also becomes the name of an object involved in the process, as with "le parking".
Sometimes it happens in English too, like "building" and "booking".
