The page is confusing...
https://www.jal.co.jp/en/dom/service...k/diamond.html
"First class domestic passengers departing from this airport on the day. Or passengers connecting to a same-day JAL flight with a first class reservation."
On the Japanese version of the page the text reads "JAL国内線ファーストクラスにて当日当該空港よりご出発または同クラスに当日お乗り次のお客さ ま" where I can only assume the "同クラス(same class)" means "国内線ファーストクラス" in the previous paragraph.
This is separately followed by the rules for passengers connecting to/from international first class. Maybe I should call them to ask...
Quick update by the way. I just spoke to JAL domestic customer service and they confirmed I can use the Diamond/Premium Lounge at both ITM and CTS. Yippee!!! I told them (in Japanese) that the language on the English site was a bit confusing compared to the Japanese site (they should make it clear that they're referring to "Passengers connecting to a same day
domestic first class JAL flight"!)
So to anyone planning a same-day mileage run within Japan - it might be a good idea to use Japan Explorer Pass to do 2-3 segments and then a domestic First Class ticket (regular fare) - that gets you in the domestic first class lounges for ALL legs