You seem to be ignoring the second half of that sentence:
> ...besides the default constants being chosen relative to the cost of a sequential read as a basis (and Postgres admits these relative values are not scientific or necessarily accurate)
The costs will always be relative to each other as defined by your constants. The whole point of the cost model is to compare plans so that the fastest one can be chosen. If you hyper-tune your constants according to your system (which I feel is probably pretty difficult to do accurately), you could maybe get something within the orders of magnitude you want.
> ...besides the default constants being chosen relative to the cost of a sequential read as a basis (and Postgres admits these relative values are not scientific or necessarily accurate)
The costs will always be relative to each other as defined by your constants. The whole point of the cost model is to compare plans so that the fastest one can be chosen. If you hyper-tune your constants according to your system (which I feel is probably pretty difficult to do accurately), you could maybe get something within the orders of magnitude you want.