I didn't realise it had that limitation, then again my only exposure to Makaton was with dealing with a friends autistic kids.
It's not really a limitation, more an indicator that they're completely different things. Sign languages are just that - whole languages which are manifest in sign rather than vocalised words. Makaton on the other hand is like a supplement to verbal languages which already exist. It isn's intended to exist or function as a language in its own right.
So your friend's autistic children might be having difficulty acquiring spoken language, maybe because their social communication difficulties don't allow them to attend to peoples' faces and the fleeting sounds coming out of them. Makaton attaches a visual support (the sign) to the word to provide a more accessible, (temporally) enduring representation of the concept which is easier for the child to learn to understand and use.
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moto