After re-reading the well-written Type 90 prototype suggestions by Tasty on the old forums, I think you are correct.
I don’t remember where I read about it being able to fit in the Type 90, but perhaps it is just a mix-up referring to the downsized 120mm version found in the second prototype.
A bit of late news, but Japan is trialing mothballing of some of their retired equipment such as the Type 74. A trial has been set for next year, and if results are promising, may continue on a larger scale.
Vehicles include the Type 74, Type 90, and (supposedly M270) MLRSs.
Some have theorized that it could be for use in defensive positions at the Tottori Sand Dunes, but maybe it’s just as likely to be for overseas deployment.