The theory he ate a Cerberus DF is highly unlikely to be true.
If he had eaten a fruit, they'd surely have known; when they realized he had absorbed the Gura Gura No Mi, they'd simply have said he already had a fruit prior to Yami Yami No Mi, this is not so far-fetched.
Their reaction implies otherwise, and they still go on to emphasize eating 2 DFs is impossible; so it's really unlikely that this Cerberus DF theory is substantial, although it is good speculation nonetheless.
Except Blackbeard was able to fool everyone into believing his anatomical structure was not as a result of a previous DF; the possibility of this even being remotely plausible is so slim.
He was in the crew for so long, someone must have had him jump into the sea to confirm his story; Zoro & Luffy were able to sense this abnormal aspect just within 2 encounters, WB without a doubt would have picked up on this & is more likely as Marco knew about it prior to Teach leaving the crew.
People who lived with him must have been even more mystified and carried out some investigation or secretly monitored his activities. No one wants to be nakama with someone they can't trust and Teach was definitely trusted; Ace even asked for his approval in being 2nd Division commander as he was there much longer than Ace was.
