So basically here it seems the text is not referencing the omniscience of the Buddha but rather the ability to see reality....the darkness in regard to everything, in regard to all knowable objects, is destroyed by him or for him...
..."Darkness", i.e., ignorance or not-knowing, for ignorance hinders the seeing of things as they are.
Ignorance in this instance is not a lack of knowledge, but it is an obscuration to viewing reality.
Pratyekabuddhas and Sravakas (and here he seems to be referring to Arhat) on the other hand:
Undefiled ignorance... So they have overcome defiled ignorance as defined above, but still lack the ability to know the 1. morality (sila), concentration (samadhi), super knowledge (prajna), liberation (vimukti) and cognition-insight of liberation (vimukti nanadarsana) of a Buddha.have not destroyed the darkness in every respect, for the
ignorance in which defilements are absent or undefiled ignorance
is active or manifests in them; they do not know:
1. the qualities [dharma] belonging [uniquely] to the Buddha
2-3. objects very distant in space or in time
4. the infinite variety and divisions of things.
4. Seems to refer to a lack of omniscience.