What was the scope of Bahá'u'lláh's knowledge?

Bahá'ís put no limits on the knowledge of the Manifestations. Yet Bahá'u'lláh, in one of his holiest books, has admitted to being unable to know a subject without reading it:

As We had frequently heard about him [Haji Mirza Karim Khan], We purposed to read some of his works. Although We never felt disposed to peruse other peoples' writings, yet as some had questioned Us concerning him, We felt it necessary to refer to his books, in order that We might answer Our questioners with knowledge and understanding. His works, in the Arabic tongue, were, however, not available, until one day a certain man informed Us that one of his compositions, entitled Irshadu'l-'Avam, could be found in this city.

Bahá'u'lláh, Kitáb-i-Íqán, pages 184–5

What, then, did it mean when Bahá'u'lláh or 'Abdu'l-Bahá spoke so confidently on science, history, or even current events?