The linked article is pretty explicit about how they calculate it. A detailed explanation is literally the first section.
And as they explain, they ask people about the actual problems they've had in the last 12 months with their car; nothing soft or fuzzy or relative to expectations at all.
If I ask you about your experience with a Rolls Royce you'd probably mention in the door rattles a little bit between the speeds of 55 and 60mph. If I ask the same question about an entry level Toyota I doubt people would think anything of it or mention it. This is especially impacted if you're used to buying cars that cost $75k (a lot of Tesla owners) versus used to owning preowned cars bought for $10k
And as they explain, they ask people about the actual problems they've had in the last 12 months with their car; nothing soft or fuzzy or relative to expectations at all.