I am not against and nor in favor of the belt treatment, although it sounds VERY PROMISING and there is no harm in trying once on the exterior belts like AC, Steering and Fan. Of course not with timing belt unless fully convinced.
I'm using WD40 on all my belts first squeak I hear and to be honest, I think I have delayed the change period. I do exactly what above video showed but with WD40 when the vehicle is off, I spray generously as far as I can reach and wait for 2 minutes and start engine and let it run for 15-20 seconds and spray again with engine off - do it 3-4 times until belts become moisturized and all squeaking has gone.
I'm using Pajero for 7 years now - only long ownership car, rest all I flip within 2 years. First two times I changed all my belts within 2 years time, and now since 3-4 years, I haven't changed them.
I know before few people jump about WD40 so here what it says it can do and yes it's composition is rubber and leather-friendly.
https://www.wd40.com/img/WD-40_2000_uses.pdf
WD-40's main ingredients as supplied in aerosol cans, according to U.S. Material Safety Data Sheet information, are:
50% "aliphatic hydrocarbons". ...
<25% petroleum base oil. ...
12–18% low vapor pressure aliphatic hydrocarbon. ...
2–3% carbon dioxide. ...
<10% inert ingredients.