There's no doubt that this is a serious incident. I'm not downplaying head injuries but there is a difference between not being aware of time and place in the immediate aftermath of a head trauma and losing the memory of your whole adult life for good.
However people are taking uncorroborated accounts of what Alonso said right after the crash and making it sound like to this day he still doesn't know what year it is or what level of racing he's competeing in.
The reason these uncorroborated accounts have gained so much momentum is that the official account has been so inconsistent and generally suspicious.
From the original accident, with the first eye witness accounts describing it like he tried to pull over, McLaren going completely silent at first before blaming it on the wind a day later, announcing he was fine, saying the prolonged stay at the hospital was just precautionary and there was nothing wrong, before later saying he was concussed (while somehow simultaneously asserting he was uninjured...) and going back and forth with the official story. Combine that with the fact that the impact appeared to be far less serious than the ones you see drivers walk away from all the time.
Then there's the complete lack of any information from Alonso himself, apart from what appeared to be one very short highly scripted and carefully worded video release, and it's easy to see why people are worried that everything is not as straightforward as McLaren are making out. Respected people from the F1 media, and from within F1 itself, have questioned what happened and described it as strange.
There's no smoke without fire, and there are so many suspicious circumstances surrounding this case that it's difficult to take McLaren at face value (especially when they continue to use evasive language - when asked about the memory loss they replied that "it is not unusual" rather than confirming or denying anything or giving any details) - of course, Alonso's personal medical details are between him and his doctors, so you could say that it's not McLaren's place to reveal anything, which is true. But if there was nothing to reveal then they wouldn't need to be so evasive.
Hopefully Alonso is all clear and back racing in Malaysia, but I wouldn't be so dismissive of some of the reports coming out, purely because McLaren are clearly hiding something with their behaviour.