Actually, there are quite a few reasons they might use vinegar.
One would be that their water supply may be quite hard. It seems plants have a very hard time of absorbing nutrients in extremely hard water. (Mine is off the scale above 8.5 for ph)
Second, vinegars like old fashioned apple cider vinegar (the kind that is cloudy looking because they don't filter out all the residues and pulp etc) contains a great deal of minerals and other items that are very good for plants. HOWEVER, you do need to significantly water down the vinegar, because vinegar used straight or in too high of a concentration will kill plants very quickly.
For evidence, I found the following quote on another web site:
The following USDA website indicates that cider vinegar contains some essential mineral nutrients, including significant amounts of potassium (730 mg/liter), phosphorus (80 mg/liter), calcium (70 mg/liter) and magnesium (50 mg/liter). Compare these numbers to those for a Hoagland Solution at the bottom. Note that the USDA tables use units of mg per 100 grams. Multiply by 10 to get standard units of mg/liter.
I can't find my other quote right now, but I remember reading that the old fashioned type of apple cider vinegar will also encourage and feed beneficial bacteria and other soil organisms - which in turn will better feed your plant.
I'm just experimenting this year with using natural materials - non-animal based - as fertilizers. It is quite amazing what you can find out with a little persistent digging online. I am going non-animal because I have a horrible time keeping the critters from digging up all my plants when I use animal based material. I'm glad that happened, or I'd never have discovered the wonders of alfalfa, molasses and even dandelion greens and flowers for steeping a "tea" to feed my plants. It's only been a month or so, but my plants have reacted more positively to this gentle fertilizer than anything I've ever fed them before. But I digress - you asked about vinegar. My point is, if the gentleman is using vinegar and you see he has wonderful plants keep investigating and give it a try yourself. Nay sayers will always be there to tell you "don't bother!" - but until you try it - you just don't know what you're missing. :)