I think this is different between companies but not in the way you are suggesting.
For example, concerning the subject we are discussing in the first place, Tesla because of the specificity of their cars probably have to produce themselves a huge part of the components of the car whereas Ford, Volkswagen,... are now to a point where a huge amount of the parts of their car are similar and can be outsourced to contractors that will decrease costs by producing only one type of components in huge quantity and provides to multiple constructors (who looks more and more like assemblers nowadays).
So I agree that it is in fact not easy to compare manufacturers on this metrics but I would believe that it's actually suggesting that Tesla does a lot more in-house than conventional car manufacturers.
For example, concerning the subject we are discussing in the first place, Tesla because of the specificity of their cars probably have to produce themselves a huge part of the components of the car whereas Ford, Volkswagen,... are now to a point where a huge amount of the parts of their car are similar and can be outsourced to contractors that will decrease costs by producing only one type of components in huge quantity and provides to multiple constructors (who looks more and more like assemblers nowadays).
So I agree that it is in fact not easy to compare manufacturers on this metrics but I would believe that it's actually suggesting that Tesla does a lot more in-house than conventional car manufacturers.