Yeah the economy is larger than it is reported and a lot of official figures exclude backyard activities. Just found out that agricultural output (pepper, onions etc) grown in backyards like you mentioned don’t go to official numbers. In a place like nigeria that’s maybe 50% rural that’s a great deal of output.Most Nigerian "statistics" are guesstimates.
The overwhelming majority informal economic activity that is witnessed on virtually every street in Nigeria is NOT captured in any reliable (not in fact meaningful) manner. Furthermore, if I grow yams in my backyard and sell same to my neighbours or local community (or even consume same), exactly how does the devaluation of the Naira against the US dollar affect the "size" (and thus the output) of that economic activity (and thus the size of the GDP)?
However it affects the formal economy so it does take a hit. The devaluation makes it hard to buy fuel, imported manufactured goods like cars, fridges and inputs for your business.