If a stove has a good fan system to 'wash' the heat off of the stove body, then it probably will NOT be glowing red, and if any part is nearing red-heat temperatures, it would be the firebricks around the fire..
Even the antique stoves with a firepot exposed on the outside to the room being heated, rarely glow red, and if the firepot has a refractory lining, the pot will not glow red..
I don't know the actual temperature at which iron or steel starts to glow red, but I think it's in the mid 800* range, bright red is around 1100* I think,, I'm sure you can google it.
So there are plenty of people who on really cold nights will run their stoves with a surface temperature of 600*, and that is making a LOT of heat.. how much would depend on each make and design of stove.
The BTU figures are based on the quantity of coal being burnt, and the ability to get air to it to sustain a high burning rate..
You won't find a handfired stove with a shoebox sized firebox claiming 100K BTU, probably about 40K BTU, but a big harman Mark III or SF250?? those fireboxes are BIG and hold a lot of coal, aka Hard BTU's.. so if you can supply enough air to get it all burning hot, yep, it will probably make the advertised BTU's..
For example: an AA 260 turns it's auger twice as fast as an AA 130, twice the coal is available to burn at max output. All part of
the design.. I'll bet that an AHS 260 would have a bigger feed hole in the bottom of the hopper than an AHS 130.. again, part of supply for output.
I don't think the BTU claims are lies, that's for sure, but some may be overly optimistic but still not off by huge percentages.
Just my take on it.