Almost impossible to not be prime at 28. Don't tell me he was in his prime at 36 against a blown up Welterweight. His punch output had already dived by then.
Please, people are being call ATG's for accomplishing far less. Lomachenko comes to mind. Just search what people have said about him on this site.
Hell if I became a world champ being a very average fighter as you described I'd think I overachieved in life. Thornton did nothing with his career, beat Dave Tiberi on a cut in a fight he was losing on the cards but that's about it.
It's called an alphabet belt, I'm sure they were vacant titles too, won against nobody. Thornton was in that hotbed of middleweights in the 80s and early 90s, a real crafty front foot technician who would've beaten Eubank and Toney if they weren't totally on their game and razor sharp (and they knew it). Great wins over Tinley and Sosa and very high standard US title fights on national TV. Dangerous right hand and concrete chin. He totally out skilled Merqui Sosa (who did good with Toney and had those barn burners with Prince Charles) with short inside hitting. Thornton was ranked #1 by all four bodies when he fought Eubank.
We should talk about him as an ATG now. He didn't lose a fight across 5 weight classes until he was 39 years old. He barely had a tough fight.
I recall a Ring article covering the prospect Thornton in 87 suggesting he had the best right hand in boxing bar Thomas Hearns.
James Toney on Thornton: 'I'm surprised you haven't said anything bad about Tony Thornton yet!' 'Well, he hasn't tried to diss me. I respect Tony Thornton. He's a cagey veteran. When I was a kid I used to watch him on TV. He beat lots of great fighters before I even turned pro, So he knows his way round a ring as well as anyone. He knows all the angles, all the moves. He can be real slippery and I know he's tough. But when the fighting's over he's just a regular guy.' I thought he beat Collins, and if you judge on aggression over ring generalship then he beat Eubank, too. DeWitt had given Hearns all he could. And Thornton made every round close w/ Toney.
Everyone is different with regards to the ageing process. If an athlete is able to avoid injuries then they can peak later than the average over-trained / over-competed athlete who’s bodies doesn’t repair as fast once they’re into their 30’s. The older and more experienced one gets in any particular sport, the better they should be if physicality wasn’t a factor. Hopkins was most likely a better boxer at 35 than when he was 28.