I'm sure that early speaking/writing/reading/etc is highly correlated with high IQ's but I'd also caution that the absence of them doesn't mean the child isn't gifted. Gifted kids can be very asynchronous and/or have other exceptionalities which along with the whole personality aspect can mean some kids are more obvious than others or they are obvious in some moments but then average or behind in others. You can find stories here of kids reading at 2-3 while other kids here were barely speaking and in speech therapy at that age. Fast forward a few years and they can both test PG.
I will also say that having one kid that is one flavour of gifted also really skews your judgement of what all gifted kids look like and what is actually average. Volunteering weekly in DS's average grade 1 classroom was a real eyeopening experience a few years ago (DS is 2e so it eyeopening on both ends of the spectrum).
