Every time my granddaughter came to visit, she always said, "you always smell so nice Nana" Probably because I always have a shower every day and put on a lovely powder, Lavender , by Cashmere Bouquot, my favourite. My Dad always smelled of Old Spice aftershave.
I used to love to hug my elderly Aunt Florence who always smelled like Cashmere Bouquet! I must get some! Or Chic's essential rose!
I found many sources that support the article in the OP, but it didn't say much about poor hygiene,
like not showering regularly, and a tendency to wear the same clothes over and over - practices which may be more common among the elderly. Also, seniors are often on multiple medications, and some of those like blood pressure meds, antidepressants, and anti-inflammatory drugs can have an impact.
That's been the simple answer for most of the bad smelling old people I've known. I imagine they're probably thinking, "Well I can't see any dirt on my clothes and I haven't done anything to work up a sweat so I don't need to shower and change." Sadly they haven't considered, bacteria, air pollution, body oil, dead skin cells, etc.
My father always said we can't smell ourselves because of the olfactory fatigue thing, so we'd better just bathe everyday to make sure.
I expect I usually smell like a combination of, Windex, Clorox Ajax, Mr. Clean, Old English polish and Dawn. Sexy!