Strictly speaking the Eclipse Foundation does *not* have a trademark agreement with Oracle. We have a copyright license, but have no special rights whatsoever to any of Oracle's trademarks.
In addition to the above, any specifications which use the javax namespace will continue to carry the certification and container requirements which Java EE has had in the past. I.e., implementations which claim compliance with any version of the Jakarta EE specifications using the javax namespace must test on and distribute containers which embed certified Java SE implementations licensed by Oracle. These restrictions do not apply to Jakarta EE specifications which do not utilize javax, including future revisions of the platform specifications which eliminate javax.
This means that any Jakarta EE *specifications* that contains even a single javax namespace has Oracle-imposed runtime restrictions that may or may not be good for the community.
From the point of view of an implementer, I don't think that there would be any restrictions on claiming that a single version of Eclipse Jetty supports both Jakarta EE 8 (using javax) and Jakarta EE 9 (perhaps 100% using jakarta).
Does that help?