Couldn't it also be a perfect example of the bandwagon fallacy?

No. I’m not making absolute statements but casting doubt on one persons claims that a product is probably bad they have no experience with vs. a large number of professionals who use it as a tool every day including several organizations conducting comparative equipment tests.

Professionals are prone to confirmation bias.

Stanley, Black and Decker, Dewalt, Irwin, and Porter Cable are all tool brands made by the same company in the exact same factories. People have ran extensive "scientific" tests on job sites and on YouTube on the battery life between them. Everyone has a favorite that lasts the longest or is best under load. Aside from the metal contacts and plastic housing to make the batteries unique to each brand - they all contain batteries from the exact same two suppliers.

Serious question:

Do professionals use those brands? I'm sure passionate amateurs will use those brands (I have a drill from that list) - but pros?

I ask because I know a professional mechanic and I asked his favorite wrench brand, and he went off about various brands I didn't recognize.

Also, regarding your list, have you accounted for variability in quality within a product run?

I suspect consistency of product is part of the reason thermapen have their reputation and other brands mostly don't. You can often get cheap products that are good, but there might be a lot more variability.

Binning is a related topic.

I have worked in a position where I repaired all of these brands, and can say that at least in the last two decades Stanley Black and Decker Inc. has been very strategic with product positioning and continued support for all of their brands. DeWalt is supposed to be their professional (or probably prosumer) line that's built to last, or at least to be repaired. Black & Decker, which had been a higher-end brand before the 90s, is now pretty much all "happy homeowner" tools built out of less durable plastic and often without replacement parts available. Porter-Cable seems to fall in the middle, a bit closer to B&D.

There's a certain 20V compact leaf blower that I've seen branded as a DeWalt, Ryobi, and Milwaukee. I'm not sure about the motor or controller but the plastic casing is almost identical aside from the color and battery connector. It's also interesting to note that the B&D and the Porter-Cable 20V batteries are identical except for a plastic tab that prevents interoperability.

Same four* suppliers actually, just about every rechargeable embedded battery is made by Panasonic, Sanyo, LG, or Samsung.

Most common type/size is the 18650, used in everything from drills, powerbanks, to Teslas. Second most common type is the 21700.

A "scientific" test isn't even necessary, just tear down the battery pack, see what cell they put in there, and read the spec sheet. The big 4 battery makers will sell you a whole range of batteries in that size with varying mAh/Amp.

Googling teardowns, I found a Black & Decker 1.5A MAX Lithium simply uses a bunch of Sanyo UR18650W2 (1500mAh rated at 18A) cells, while a teardown of a Dewalt pack shows they used the more expensive LG HB4s (1500mAh rated at 30A).

There literally no reason they can't use the better cells in the Black & Decker, other than they're building it to a cheaper price point. The fact that they're built in the same factory is besides the point.