Each manufacturers' products in Japan never fully utilize components made by themselves, but use a lot of stuff from their competitors that specialize them, way more cost efficient (or even better performance) than to make everything yourself... based on this original concept, that's part of how the Japanese industry success came about... and started a trend all over the world...
For example, I've opened up an old Sony tape Walkman from the late 80's and I can tell you that the radio board is made by Mitsumi... the design could be Sony's, but they just chose to outsource the manufacturing to others. Also, the machine carries a few Hitachi ICs...
IMHO one can never say who manufacturered the product just by seeing who made the ICs on the board, there is a chance that you're right, but no always for sure tho...
BTW, Sony always had some shares in Aiwa (less than 50%) since the 80's until a couple of years ago they went over 50% and hence took control of the company, and as a result Sony forced Aiwa to make low end products or even clones of Sony ones... so mean! Cuz long long time ago Sony's products are inferior to Aiwa ones (e.g. if you played with Walkmans since the 80's then you'll know what I mean)...