I think bristlemouths just make up a large share of all fish, by numbers of individuals. Lanternfish do, too. When someone says "a quadrillion", this is an order of magnitude estimate, so it could easily be 3x too high or 3x too low. The estimates seem consistent to me as order of magnitude estimates, as long as bristlemouths do make up a decent share of all fish, like >30% of individuals.

The Micronekton Wikipedia page says:

Bristlemouths (Gonostomatidae), largely Cyclothone, account for more than 50% of the total vertebrate abundance between 100 and 1000 m. Twenty-one species of bristlemouths have been described globally. Lanternfishes are the secondmost abundant marine vertebrates, having diversified into 252 species.^[24]

Futhermore, among pelagic fishes from 0-5000 m in the Sargasso Sea, northwestern Atlantic Ocean, “[t]he bristlemouth, Cyclothone braueri, dominated the catches both above (47%) and below (41%) 1000 m” (Sutton et al., 2010).

The number of bristlemouths has been estimated to be in the “hundreds of trillions — and perhaps quadrillions, or thousands of trillions” (Broad, 2015), so 10^14 to 10^16.

I estimated the number of lanternfish to be approximately between 10^14 and 10^16, based on a “total global biomass of 1.8 to 16 gigatonnes, accounting for up to 65% of all deep-sea fish biomass” (Lanternfish - Wikipedia, based on Hulley, 1998, pp. 127–128, ISBN 0-12-547665-5) and an average weight of "two to six grams" (Blacow, 2015).

This is all from my piece Which animals are most affected by fishing?

On whether they're R-strategists: ecologists may have moved on from that classification, but either way, bristlemouths are fish, and almost all fish species, as far as I know, have many offspring (at least hundreds?) and high juvenile mortality rates (>90%?). Bristlemouths are forage fish, so food for predators. But they also probably eat zooplankton, like copepods.

Did you see this blog post from Wayne Hsuing? https://blog.simpleheart.org/p/the-mass-extermination-of-animals