I think this is mainly a textual formatting problem, the idea behind the spec is to modify ANY imported file with the prefix.
This is logical (I think) since there are very few unofficial parts that have only a single .dat, so you have to import the whole thing recursively for it to be useful. In such a scenario adjusting the type1 lines seems implied.
Doing all this while leaving the name tag alone gives unpacking software the possibility to restore the original file names.
But you are right the spec could mention all this better.
This is logical (I think) since there are very few unofficial parts that have only a single .dat, so you have to import the whole thing recursively for it to be useful. In such a scenario adjusting the type1 lines seems implied.
Doing all this while leaving the name tag alone gives unpacking software the possibility to restore the original file names.
But you are right the spec could mention all this better.