Frankly, I thought the article was miles off the mark.
For a start, Labour isn't haemorrhaging. That would imply it is either losing members or votes. It now has a record number of members, and to sneer at those who have joined because of Corbyn makes no sense. Of course people will join a party if they feel strongly that the leader is expressing their views, and will leave if they feel disillusioned. (Unless you think thousands of people across the country are engaged in some sort of left-wing conspiracy, in which case you are beyond rational argument!) As for votes, we won't really know until a General Election. A third of Labour voters opting to Leave means nothing as far as support for the party is concerned. None of the parties have been wholly for or against the EU, and there have always been Labour supporters, on both wings of the party, who have wanted the UK to leave. It would be equally simplistic to assume that a Tory voting for Brexit was simply voting against Cameron or for Boris. I'm actually quite surprised that two-thirds of Labour voters went for Remain, given that the issue cuts across party politics.
As for "swathes of Labour MPs" failing to understand their core constituents, I'd say that most of them think their voters hold conservative values, hence their determination to emulate Tory policies in a milder form and their panic at having an actual socialist at the helm. Whether they are right or wrong remains to be seen. I remember reading a survey, a couple of years ago, which found that the general public were actually more radical than the political parties on most issues except immigration. Unfortunately, the Leave campaign centred on immigration, which should have given them a pretty easy win. Surprising, really, that they only managed a winning margin of less than 4%, given that migration is such an emotive issue.