Yes, this is a reasonably common strategy. It's how Cassandra's batch and group commit modes work, and Postgres has a similar option. Hopefully NATS will implement something similar eventually.
In addition to the SATA port multipliers, they'd need actual SATA PCIe cards. Basically nobody makes a motherboard with onboard SATA that supports port multipliers.
That's the price being paid for the storage, but is it actually covering the cost of providing that storage? Or is it just 100-300 people who thought "Huh, neat, I'll toss a host online and see how it goes" ? I'd lean towards the latter and assume those storage costs are heavily subsidized by a few people satisfying curiosity.
Even if it had slots for the splitter cable, Intel and AMD onboard SATA explicitly doesn't support port multipliers as far as I know.
You can buy PCIE SAS cards that do for relatively cheap, but then you have to find board with enough PCIe slots. Easy enough on the "gamer" boards but if you want ECC (and you probably do, for storage) and IPMI (you probably do, if you have more than a few dozen servers) your options get much more limited. Other than 1 or 2 Asrock Rack boards, you pretty much have to move into Epyc 7000-series or Xeon Silver or above. Often dual-socket on the Xeons to get a board with lots of PCIe.
In theory something like an Epyc 3000-series with lots of PCIe or onboard SATA that supports port multipliers would work great, but I don't think anyone actually makes that.