The Dauphiné can be misleading.
In 2008 Carlos Sastre finished 20th, twelve minutes behind Valverde.
In 2014 Nibali finished 7th after being dropped uphill several times.
The only certainty is that Jack Haig always finishes 5th or 27th.
Okay... but Wiggins, Froome, Thomas and Vingegaard all won it before winning the Tour. Looking at the last 15 years or so.
Yates second last year, 3rd in the Tour.
Vinge second in 2022, won the Tour.
Evans second in 2011, won the Tour.
Contador second in 2010, won the Tour (on the road at least).
Thats not even going through how many riders that finished top 10 in both the Dauphine and Tour the same year. I think it would be pretty common.
I would say it is a pretty good indicator of where a rider is at and that with a good performance it is likely they will do well in the Tour.
Saying that it is "misleading" is false imo. It more so depends on how they are building up and where they are at in their progress. What type of rider they are. Who they are.
Sastre for example was a GT-specialist. He hardly ever did any results at all in one-week races. 2006-2009 he was on a great run through every GT, so his win in 2008 was very deserved and he was one of the best at the time.
Nibali had some luck that everyone else crashed out or got injured, while also finding great form himself... but his biggest rival for the win was Peraud and a still relatively young Pinot. Nibali was also at his best in almost every GT during this period and had been since 2010. In 2014, he very much peaked for the Tour only and got a perfect scenario after a pretty mediocre season outside of the Tour.
Both of those are bad examples to that claim it can be "misleading". Both outliers and you are always gonna be able to find a couple of those, but claiming two great riders like this are outliers is also "weird" imo. Especially how proven they both were and that despite being mediocre before the Tour was seen as two big contenders.