From my findings, the underlying bond holdings RIC codes of LP68089870 are not accessible via Eikon Excel at this time. Therefore, if Eikon Excel is unable to access it, it is unlikely that Eikon API can get it.
However, it is accessible via Eikon App. To get Bond RICS using Derived Holdings from Fund Overview in Eikon app.
However, you can contact the Eikon support team from confirmation.