Sure, P=I²R but only when you control the current. And P=U²/R when you control voltage. But USB-C can only give you 9V, 12V, 15V or 20V (depending on source, several of these steps might be missing), or 28/48V with EPR.
Given that resistance is mostly fixed you cannot reach the maximum rated power, unless your tip is precisely matched for any of these fixed steps. And PWM does not work with some power sources because of sensitive overcurrent protection.
So you're saying that the iFixit iron can reach maximum rated power (100w) from 9v, 12v, 15v, and 20v, and 28v/48v? Or some subset of these?
Or what, exactly? What function does a buck converter serve in this application?
You'll have to spell it out for me, because right now using a buck converter in a portable USB-powered soldering iron sounds like a solution looking for a problem to solve.
Yes, this allows ifixit's iron reach maximum power the supply can provide (usually requesting maximum supported voltage and sinking up to maximum allowed current), regardless of current temperature and/or tip model.
Most of casual users of usb-c irons do not care about it, but if you ever used professional stuff, these usb toys appear to be deficient in comparison.
That all makes sense, but: Isn't a pro likely to be inclined to keep the most correct portable USB brick with their portable soldering iron?
They are universal, after all -- at least downwardly.
And what may be some examples of a "pro" soldering iron that uses a buck converter (or transformer taps or whatever) to be flexible to a variety of input voltages?
Given that resistance is mostly fixed you cannot reach the maximum rated power, unless your tip is precisely matched for any of these fixed steps. And PWM does not work with some power sources because of sensitive overcurrent protection.