The embarrassing and comical saga with this so called "car jurno" Steve Sutcliffe continues with this childish explanation on his blog:

G I'm sorry
New21 hours 30 min ago
Regards the 5g cornering claim, it’s a long and not especially compelling story, culminating in this official response from McLaren, who just emailed me saying; "We have a slightly embarrassing confession to make, and admit that the information quoted is incorrect - this fact was take from details regarding our Formula 1 car, and not the P1. So these figures relate to the F1 car instead. Sorry for this confusion. For information, the P1 can withstand lateral forces of up to 2g when cornering and braking." I'm sorry, too, that I didn’t spot this dear reader; I should have done. No excuse. And when I say that the P1 feels “as quick as anything I’ve ever driven including a Honda F1 car” I am exaggerating, of course. Slightly. But not by much. The P1 is much, much faster than I was expecting it to be, basically, and maybe it has frazzled my judgment somewhat in the process! Kind regards Steve 

Some conclusions:
1. Mclaren gave him the wrong piece of press release - very, very professional IMO ;
2. SS wrote what he get without even reading it, haha;
3. exaggerating comparisons, but not much? How much is not much? 

From the whole review at the end of the day, maybe the date is the only clear fact...