I am not sure how old that article is and the author does not site his sources. It may be that when they first came out, they were required to be worn. My reference has been the USCG and Iowa DNR sites.
The following links only reference that it must be approved, accessible and serviceable to be counted.
http://uscgboating.org/recreational-boa ... jacket.php
Page 12 of the Iowa boating laws also makes no reference to having to wear them.
https://www.boat-ed.com/assets/pdf/hand ... entire.pdf
Also, I cannot find anything in the Mustang manual that says it must be worn.
The following is from the Mustang manual - it states that inflatable must be worn on Commercial vessels to count but makes no reference to that in the recreation section.
______________________________________________
Recreational - This Inflatable PFD is approved by the U.S. Coast Guard (USCG) for use as a Type III Personal Flotation Device (PFD). It is not approved for water skiing or other high impact, high-speed activities.
This Inflatable PFD was designed to be more comfortable and less restrictive to wear than inherently buoyant PFDs. When worn, used, and serviced according to this owner’s manual, this Inflatable PFD can greatly increase your chances of survival in the water. Not recommended for non-swimmers or weak swimmers. Users of inflatable PFDs must be at least 16 years old.
Commercial - Models MD2014 and MD2016 are U.S. Coast Guard approved Type V inflatable PFDs, approved ONLY WHEN WORN for use on uninspected commercial vessels less than 40 feet in length not carrying passengers for hire as a substitute for a Type III PFD when used in accordance with this owner’s manual.
Conditions for Type V Approval in commercial applications:
1. The Inflatable PFD must be worn before getting underway and whenever above deck and not within an enclosed space to meet the carriage requirement.
2. The operator of an uninspected commercial vessel is responsible for servicing and maintaining the Inflatable PFD in good serviceable condition in accordance with this owner’s manual. See Care and Maintenance Instructions for servicing information.
3. The operator of an uninspected commercial vessel is responsible for providing each person onboard with the safety information required for use of the Inflatable PFD. See page 1 for information.
________________________________________________
Main point is that the best approach it to have any life jacket on vs just accessible........