Having never seen a dob of batshit, at least not so far as he is aware, Alf is none the wiser about the depth or dearth of David Beckham’s intelligence.
But he is aware that “batshit” is a measure of a bloke’s braininess employed by The Boss.
He might have said as thick as guano, which is the excrement of seabirds, cave dwelling bats, and seals.
He might further have pointed out (as your learn here) that –
Guano manure is an effective fertilizer due to its high levels of phosphorus and nitrogen and its relative lack of odor compared to other forms of organic fertilizer such as horse manure.
Dunno if it is better to be as thick as batshit, apeshit or pigshit.
Or as thick as guano.