It means sleep was the cause of lunch being missed. No more details are provided or implied. Off the record, one could guess that the speaker didn't want to remark on oversleeping due to the negative connotations, and is instead leaving the answer vague in the hopes of don't-ask-don't-tell. Depending on the audience, this may also be exactly enough information to be unambiguous with the fewest number of words; a friend may have had prior experiences that set the default interpretation and context of mid-day sleep.
I think the implication of dumb is there, but it's supposed to be transferred to the subject of the sentence. As in, the person is mocking himself for being lazy.
Lossy compression always uses heuristics to guide compression and decompression. In this case, the heuristic is 'my brain works similarly; why would I have written this?'
TL;DR: Don't mistake dumb for compression.