Hi Jeroen,
I would suggest open up a PMR and that is the official way to confirm any issue that you may have with respect to the note that you had referenced.
I would take this opportunity to clarify some terminology. 6.1.0.23 is the complete version info and the fixpack is "23" here.
And this note that you had states clearly that "Java SDK 1.5.0 SR9 Cumulative Fix was fully tested with WebSphere Application Server V6.1.0.23.". It does states that this CF would result in "The cumulative fix for Solaris will upgrade to 1.5.0_17-b04".
The statements imply that IBM would support this JDK version for WAS 6.1.0.23 that it is supported via the formal support process.
Would this JDK fix the EJB issues for you is another question all-together? The error java.lang.NoSuchMethodError doesn't sound like one which is going to fixed as a result of this JDK Cumulative fix. I haven't looked at the error in detail though.
Caveat: I work for IBM but this opinion is mine and does not in any way reflects that of my employer.
HTH Manglu