He's confident because in many ways it could be true.
If MS have a faster CPU then, by some metrics, the XSX is the more powerful console. If it has patented VRS. If he is confident about Direct X he can also argue that when you combine software and hardware they are the "most powerful".
If you are Phil Spencer and you have an XSX with a 10% weaker GPU but a 10% stronger CPU, and specialised rendering software/hardware you are confident in with DirectX and patented VRS what do you say when you are asked if your console is more powerful than PS5?
You say exactly what Phil Spencer has said. Exactly how he has said it.
All I'm saying is that taking Phil at face value as meaning the XSX has a higher TF GPU than PS5 is a large leap of faith, especially when we now know that the targets haven't changed and all other sources suggest that PS5 has the advantage.
It may well be the case that the XSX has a better GPU than the PS5, but Phil Spencer's statements aren't strong evidence of that. They come from a biased source with a dog in the fight and are worded in a way that can be spun any number of ways to be "true" even if the PS5 has a higher TF GPU.
So personally I would treat Phil's statements as support for the evidence that the consoles are very very close in performance because he probably wouldn't make them if that wasn't the case. Is it evidence in support of some big change in the relative power of the GPUs from the Reiner/Klee/Voldemort leaks? I don't think so - especially when Klee says nothing has changed and he has the final specs of the consoles.