Unfortunately, without any conclusive audio of Langan speaking at 6 months we can't really prove whether he could or could not speak. However, it is possible, though unlikely, that he could speak very simple words that early. Such words would depend on fast physical development rather than high IQ, and wouldn't mean he 'learned to speak'.
The first step on the path to language that (pretty much) all babies go through is called babbling. Although the exact connection between babbling and actual speech is not fully known, the different stages of babbling have been consistently observed in infants worldwide.
The relevant babbling stage here is that at 6 months old most babies can reliably produce the same sounds repeatedly, such as the sound 'ma' to say 'ma ma'. Before this point, babbling is mostly random/inconsistent. The reason for this is because a baby's larynx is initially high in the throat and descends/begins developing in their first year of life. About a month after this is when their vocal system has developed enough to produce multiple different sounds in one breath. These abilities to produce and repeat sounds rely on physical development, not intelligence.
Therefore, unless Langan's larynx descended early to allow his vocal chords to develop a bit more, at 6 months Langan would have only just gained the physical ability to make repetitive sounds. At 7 months, he'd finally have the ability to produce several different sounds in one breath. Without this ability, coherent words don't seem possible: articulation requires physical control, not intelligence. However, although this means that he may have been able to reproduce very simple, single syllable words at 6 months old, that is both far away from 'learning to speak' and not at all uncommon.
