i went away on a girly weekend when dd1 was 18mos and dh had taught her the alphabet.
it was like a party trick. he had a great big bag of wooden letters tipped all over the floor, and would call out letters (or sounds) randomly and she would dig through and find the right one. it was... interesting...
without a doubt she was linking the letters and sounds written form, and was able to sort the whole lot and locate the correct one. i thought he was stark staring bonkers tbh, but i suspect most tots could do it given the opportunity. (i was only away for a weekend lol)
she's 11yo now. a very hard worker, so in the top sets/ recognised in the regional gifted programme, but isn't really that intrinsically clever, just has a good work ethic.
the other two (younger, natch) weren't taught the alphabet at 18 mos (for a start dd2 has cp and didn't talk until about 4 lol - at 18mos we were still working on sitting and trying to get her to breathe and swallow in the right order)
they are both much more able than dd1 tbh. ds1 could add coins, count change, and work out four or five different ways to get to any given number (below about 30) using multiplication at 3 - i was quite impressed by that, as it implied some sort of thinking, rather than just recognition.
early alphabet is a neat trick but probably means bugger all.