Ceiling and delivery
The most useful framing is that machinery determines what is possible and the driver determines how much of it is realised. A brilliant drive in a mediocre car can look like a mediocre result, and a routine drive in a strong car can look like mastery. Position alone cannot distinguish them.
The comparison worth making
Comparing team-mates is imperfect — setups and luck differ — but it holds the biggest variable roughly constant, which no cross-team comparison does. Over enough events it is the closest thing to a controlled experiment the sport offers.
What actually convinces me
The strongest evidence of driver quality is performance that travels: a driver who is quick in different cars, at different circuit types, with different engineers. That pattern is hard to fake and hard to explain away, and it is worth more than any single spectacular afternoon.