You’re right, but we still have some clues: TPM is the first film released in Dolby Digital Surround EX in theaters (this audio format was specially created for TPM).
The TPM Laserdisc is also the first LD to feature a Dolby Digital Surround EX track.
So I guess that we can reasonably think that the LD mix was not redone (it is a particular format, on a LD that was released only in Japan, with few copies, and we already knew that the LD format was at the end of its life, so why would Lucasfilm have lost time and money to redo a mix in in such special circumstances ?).

So we can not be 100% sure, you’ re right, but if I had to bet, I’d bet that the LD contains the theatrical audio AC3 track.