As the names are fixed to 8.3 there is no such thing as length detection/indication for the string.
DOS couldn't have copied what lots of SQL databases did, since DOS was being written around the same time as the first SQL databases.
My guess would be that, like the three character extension itself, this came from CP/M, not SQL.
As the names are fixed to 8.3 there is no such thing as length detection/indication for the string.