He won GCs in races or finished on the podium long before his "breakthrough" at 29. He had a few top 10-15s here and there, in a few other races. You also have to remember he worked for a leader in many races. He would do some great domestic work and told to let go and soft pedal, when work for the day was finished.As in super-peaking, sure, but how many others showed no aptitude for GCs whatsoever until they were 29?
Through 2010-2015 he got better and better. Minor steps each season. It was not all just classics or ITTs, if you really deep-dive into it.
2016 he won both Algarve and P-N... in 2015 he also won Algarve, 5th P-N and 2nd Tour de Suisse. Seems like a rider who could win the Tour within the next three years.
2017 he had earned himself a chance to be leader in a GT, in the Giro, after having been a very great dom for years in the biggest races and steady developing, showing he could do it but crashed out that time. Didnt finish the Tour either.
2018 he was at an age were a cyclist used to be around their peak or still at it, normally. It was hard to think Thomas would become so good "all of a sudden", obviously not "all of a sudden" if reading the above, but that can also be said about a guy like Jorgenson this season. Look at his results last season... finishing top 5 in a few very hard and difficult stages of the Tour. His best GC-result was 4th in Tour de la Provence. 13th in Criterium du Dauphine. Look at this breakthrough season and results so far this year. Isnt that an even bigger leap than what Thomas did in over a 3-4 season span?
Thomas finished 21th in Criterum du Dauphine in 2010, almost the same time as a young Thibaut Pinot.
I feel like Thomas is a case of hard work, being a great servant and then getting the opportunity... mixed with some other stuff. Because who can be really sure when it comes to this sport, but to me it is not as farfetched as you or others make it out to be.