Firstly, the text you cite isn't by Alan Weider, but is actually from the foreword to the book, and was written by Nadine Gordimer, who wasn't an historian but is best known as a
" South African writer, political activist and recipient of the 1991 Nobel Prize in Literature"
That foreword is available in full on the website of Monthly Review.
Secondly, your editing of the text in your quote is unfortunate, in that it masks some rather strange use of "quotations" by the author. The passage should actually read:
From Turin to Cairo he went, and with other decommissioned soldiers somehow got to Palestine although travel was restricted because of Zionist resistance to British occupation; on to a kibbutz where “looked at in isolation, the kibbutz seemed to be the very epitome of socialist lifestyle. . . . it was populated in the main by young people with the passion and belief that by the mere exercise of will and humanism you could build socialism as one factory or one kibbutz and the power of example will sweep the imagination of all . . . worker or capitalist.” Social theory aside, the dominating doctrine “on this kibbutz, as well on others, was the biblical injunction that the land of Palestine must be claimed and fought for by every Jew, and this meant the uprooting and scattering of millions whose people had occupied this land for over five thousand years (no mention of the collusion of the Balfour Declaration of 1917).”
That irregular use of quotes in the passage should raise some concerns. However, in fairness Gordimer never expressly states her source for the quotes or claims that the words quoted were from Slovo (nor did she claim to be an historian).
What Slovo actually wrote (in his book, Slovo, the unfinished autobiography) was the following:
Looked at in isolation, the kibbutz seemed to be the very epitome of socialist lifestyle. It was populated in the main by the idealistic sons and daughters of rich Jews who had amassed their fortunes in the Western metropolis. They were motivated by an Owenite passion and the belief that by the mere exercise of will and humanism you could build socialism in one factory or one kibbutz and the power of example will sweep the imagination of all men in society, worker or capitalist. Social theory aside, the dominating doctrine on this kibbutz, as well on others, was the biblical injunction that the land of Palestine must be claimed and fought for by every Jew. And if this meant (as it did eventually mean) the uprooting and scattering of millions whose people had occupied this land for over five thousand years, more's the pity.
As you can see, there is no mention of collusion. That was, presumably, an addition by Nadine Gordimer (or perhaps just her opinion, mangled by the poor use of quotes). You'll notice that the text is far easier to read, and makes significantly more sense, without the mangled attempts at "quotation" in Nadine Gordimer's foreword.
More generally, this question highlights the importance of accuracy when quoting and citing sources. The question stated that the oral historian, Alan Weider was quoting Joe Slovo. That was not, in fact, the case. One might expect a higher standard of accuracy in a quote from an historian, than from many other authors. Nadine Gordimer was primarily a writer of fiction.
Of course, it is possible that the poor formatting of the quotes in the foreword was introduced by the publisher (although one would probably expect better from NYU Press). However, once again, even if that were the case, one might expect that an historian would take greater care to ensure that such errors were rectified before publication than another author who had simply been asked to provide a foreword to a book.
When quoting a source (in a question or an answer) that source should always be quoted correctly and be properly and accurately attributed.