You're conflating 2 different ideas.
Nobody disagrees that the Conservatives are on the right, and Labour on the left. EDIT: This is almost the definition of right and left in UK politics.
Corbyn is on the extreme left of the Labour party (as he has been since 1983), and the draft manifesto reflects his team's views.
May is NOT on the extreme right of the Conservative party, and the manifesto will probably reflect her views, not the extreme right.
No this is not what I am saying and it wasn't what I was arguing with either.
The Labour party manifesto is not an extreme left one by any sensible measure. It can only be defined as such if you take the centre to be somewhere off to the right. It may be further to the left than Blair but that's fairly meaningless.
On the other hand, the Tories
are proposing some fairly extreme right stuff and May has been a cheerleader for some pretty extreme stuff in the past. She and her party are further right than Labour are left currently. By that measure if Labour are too extreme left to be elected then the Tories should be too extreme right to be elected also. But they aren't.
The whole thing just collapses back into the idea that you can't change what the electorate think so you need to just pander to them. That any left of centre policies are too difficult and challenging for the electorate and should be abandoned to win votes. But all that does is exacerbate the problem in my view.
We've been through that cycle. Blair shifted the whole spectrum to the right by abandoning the left and got himself elected. Well done. Then he went away. But the shift in spectrum didn't. So now the 'centrist' position is a moderate right one. And the 'moderate right' of Theresa May is actually fairly well along that spectrum short of outright loonies.
On the other hand anything moderate left is now considered extreme lefty lunacy.
Blair looks like he has killed the Labour Party and potentially left-wing politics in England and Wales. Even if you were to find a clone of him to lead Labour now they'd still be in trouble. And that's what people seem to be failing to see.
There is something more fundamental going on here and pointing the finger at Corbyn is missing that. Sadly there doesn't seem to be anyone within Labour who is up to the job of addressing this issue and finding a way through it.