Well, since you asked, my background for 60 years is electronics and high performance computing including 12 years at Tektronix measuring and characterizing every type of cable and connector you can imagine from FFT BW measurements for Corning fiber to high speed sampling to characterize connectors for Western Electric and Amp. I also hold an amateur radio extra class license and formerly held an FCC 1st class license (with radar endorsement), so I “speak” RF as well. On the other end of the spectrum, I spent several years designing power distribution and UPSs, and have spent a ton of time with Telcos teaching them to troubleshoot networking and cabling issues (Tektronix built networking test sets and TDRs).

Having said that, there are many, many cases when conductor size matters - power transmission, RF/coax, etc., but HDMI, like low level audio, etc. isn’t one where it matters unless we’re talking longer lengths or extremely small diameter, and in those cases, the protocol spec for max length (timing) drives you to “active” cables. First, as you’re surely aware, the HDMI interface is operating at high impedance and very small current, so whether 20 feet of cable is 1 ohm or 0.5 ohms, the voltage drop is negligible. Second, the size (OD) of most cables is driven by the insulation, not the conductor size. Thicker packaging may appear to be a higher quality cable, but when you cut it open the conductor is the same size as the thinner cable - just marketing hype. Third, the BB salesperson was selling the thinner cable as an advantage, so exactly the opposite of your contention that larger conductors matter. And finally, just as an example, what size conductor is used in CAT5 or CAT6 cable that is often run for a hundred feet or more? 22AWG max, but more typically 24-26 AWG. Coax or electrical transmission - absolutely, but HDMI?The current is just too small to matter.

Edit: cables up to 5M (16’) can use 28AWG wire and meet specs and cables using 24AWG can meet specs up to 15M (~50’). For reference, 24 AWG wire is .020 dia. with resistance of 25 ohms/1000 feet or .025 ohms/ft. 28 AWG is .0126 dia and 64 ohms/1000 feet or .064 ohms/ft.