Possible answers
1: The Empire doesn't know Kuat is building A-Wings for rebels
2 Kuat doesn't know that it is building A-Wings for rebels. They could be purchased by a rebel front company or by planetary defense forces then given to the rebels.
3: The Empire doesn't dare risk retaliating and driving Kuat into full scale rebellion. I'm sure Kuat turning against the Empire is high on the list of worst nightmare scenarios of many Imperial officers and officials.
Honestly I don't see why anyone is so shocked at the idea of a major corporation hedging its bets and making sure that in the unlikely event the rebels win they have a reason to not hate Kuat. In Legends there were several major shipwrights supplying ships to the Alliance under the table and others, including KDY, supplying ships to both sides of the Clone Wars. Why should the new canon be different?
You missed option 4
Some fool that has no clue just assigned one of the most well known ship yards name to it.
This just stinks of lazy canon making to me.
How hard would it be to put in a little effort to integrate the original canon with the new?
Just call it something other than the RZ-1
