Lend Lease B-25s were used by the Soviets right untill the end of the war and thats not at all surprising as it was a very robust design and and all round great medium bomber. Over 800 were delivered of all types. It would not have been unusual at all for the Luftwaffe to have encountered them in 1944. That being said many first hand accounts of combat fliers are filled with errors. Sometimes they misidentified aircraft and/or used incorrect names or designations for them so you always have to take that into account. Enemy combat planes did not have nice neat labels on them that included sub variant info.
Did the pilot really engage a B-25? who knows. From the sounds of it I would say he encountered a twin engined bomber of some type and incorrectly referred to it as a B-23. Either as a typo or mistake of identification. Many western types were sent to Russia and the Luftwaffe didnt know in advance every possible variant they might see so misidentification would have been commonplace especially in the midst of combat.
