My assumption was that a portion of Excalibur's crew did die (probably more than the 53 dead aboard the Lexington), but the majority survived and were trapped aboard. As we've seen in other eps of TOS and TNG ("For the World is Hollow, And I Have Touched the Sky" for one, "Heart of Glory" for another) it is possible in the STAR TREK Universe for life signs to be masked aboard a space vessel, particularly one that is damaged/emitting radiation.
So, it's entirely possible, given these other precedents, that the majority of the Excalibur's crew was trapped aboard the derelict ship, and the Enterprise's scanners were unable to detect survivors unless the Enterprise either scanned at extremely close range (never shown to be the case) or actually boarded the Excalibur.
While evacuating the Excalibur is an interesting thought, it wasn't necessary. Excalibur's crew could have been trapped and isolated from all other ships until the war games crisis had been terminated.