This makes no sense to me. If the concern is keeping China out, shouldn't the agreement allow subsidies in equal amount to spur the development of new cutting edge airliners that China couldn't compete with?
This has just about gone full circle, years ago the US and the EU had an agreement on large regarding civil airliners, Airbus started to get too big under that agreement so the US pulled out of the agreement and took the EU to the WTO for following what was in their agreement.
Prior to the deal being canceled the US was sanctioned a few times regarding dodgy tax avoidance on aircraft during delivery.
In reality what was happening in recent times is governments like Japan were giving major subsidies to local manufacturers to build large packages for Boeing aircraft, if you look at reduction in US content in their new aircraft most of the shift went to Japan and Korea. The US never took those countries to the WTO when they were the ones directly responsible for the downsizing in Boeing as those subsidies kept the completed Boeing product competitive.
This all worked well until Airbus setup multiple production lines in the US, now Airbus can go to the US government and ask them to take Japan and Korea to the WTO over their subsidies as the subsidies hurts their domestic production in the US. The EU could not take Japan or Korea to the WTO as the parts were not being made for EU aircraft.
This new agreement essentially is another iteration of the earlier agreement on large civil airliners. The big difference this time is that Airbus is now both a manufacture in the US and the EU, the US government cannot take a future position that benefits only Boeing over Airbus, it will need to treat the domestic production equally.
“Don't be a show-off. Never be too proud to turn back. There are old pilots and bold pilots, but no old, bold pilots.” E. Hamilton Lee, 1949