Not *only* holders. But roughly 2/3rds of specifically True Amarr capsuleers are nobility (assuming random spread of background choices, in practice I imagine it is more lopsided). So roughly 3/9ths of Amarrian capsuleers should be nobles (the extra 1 comes from non-True Amarr nobility). A much smaller percentage of that would be actual holders, which means they are certainly both over-represented among RPers and underrepresented in the larger player base. This is reconcilable IC by suggesting that bona fide holders would be the least likely candidates to go 0.0ing, so they are more likely to still be connected to empire happenings, i.e., to RP.
But in the Silas and Gaven case, it's worth pointing out that the distinction between holder as a class and holder as the official title was much less clear in those early days.
These days we talk about someone being a holder as specifically being a title. So you can be noble while not being a holder. Back then I thought of holder more as like a caste, so nobility and holder were synonymous. I don't think I would have made Gaven a head of household type holder in the modern RP sense if I was making him today, but that sort of got grandfathered in over the years as things crystallized.