It might be illegal, but it happens, and the article includes one very concrete example of that.
Even if everyone agreed to use your IOU from the article the tax office is going to have quite a bit to say on the subject and probably start launching audits. And if the IOU start getting traction they are likely to trigger police raids and maybe a legislative response (look at how Facebook dropped their Libra for example). That seems substantially different from the IOUs the banks can create.
I wouldn't use bank-issued IOUs if I thought I had a choice. I'm pretty sure I'm forced to by government policy.
IANAL, but the IOUs created by Full Tilt Poker were also legal to use. When the government eventually pressed charges in the debacle, no charges were pressed against regular players of the poker site, including those who used these IOUs for small economic activity. If using these IOUs would have been illegal, then surely the government would have pressed at least some charges after spending years investigating the case?
But anyway, even if _using_ the IOUs was not illegal, creating them like a fractional reserve bank surely was illegal. So you definitely have a point that the regulatory environment creates a special status for the legal tender of the country.
When I give a Amazon gift-card to someone, it is an IOU against Amazon, and most people will gladly accept it as money.