The first thing I should point out, just as someone who wants people to read rigorous history, is that it is unwise to read Runciman's A History of the Crusades for a good understanding of the crusades. Although his work is brilliantly written and important to the historiography of the crusades as a series that defined crusade studies for a generation, the field as a whole is pretty keen to move on from his work because he was not as academically rigorous as we would like. His books are decades out of date, and have been surpassed by a variety of more modern histories that, though not quite as well written, are better overall. I'd recommend God's War by Christopher Tyerman over Runciman any day, even though Tyerman's writing is a bit esoteric.

I think it's worth explaining why he thinks of the Fourth Crusade as the last of the barbarian invasions, a comment he makes both in his books and in a 1987 documentary. And yes, the obvious response to his statement is "hang on, WW2" so I think it's worth not only summing up what other historians think of that view (which isn't much), but also why he would think something so drastic. No modern historian buys into that, nor did other historians at the time (though there were very few historians of the crusades in the 1950s). The most ardent critic of Runciman was Jonathan Riley-Smith, who went on to also be the most impactful crusade historian of his generation. He took particular aim at Runciman's characterisation of the crusaders as brutish, selfish thugs and argued that it was not grounded in the historical evidence. In his books and specialised articles like Crusading as an Act of Love, he systematically dismantled Runciman's view of the crusades as one in which simple barbarians overran great and sophisticated eastern empires by elevating the image of the crusaders from thugs to earnest and holy warriors. Historians since Riley-Smith have argued that he went too far in the other direction (just as Runciman was very into the Byzantines, Riley-Smith was very sympathetic to the Catholic sources, and was even a member of the Knights Hospitaller), but generally speaking the vast majority of historians have taken the Riley-Smith view of Runciman's work, because the evidence is there to support it while Runciman's view is not based on much.

So why did he think this? The first thing to note is that Runciman was a colourful guy. In his early academic career he travelled widely and fell in love with the cultures and history of the eastern Mediterranean. He was appointed as the professor of Byzantine Art and History at Istanbul University in 1942, mostly because he was one of very few people studying the Byzantine Empire at the time, but also because the British Ministry of Intelligence wanted a guy in Istanbul to schmooze with German diplomats and Runciman fit the bill. Over the decades he did piano duets with the last emperor of China and claimed to have read tarot cards for the king of Egypt. This is all to say that he was a bit eccentric with a flair for the dramatic, and a lot of the things he said have to be treated with that understanding. They way he viewed and wrote history was a product of this. As he says in the preface to A History of the Crusades:

"It may seem unwise for one British pen to compete with the massed typewriters of the United States [a panel of American historians was writing a history of the crusades at the same time as Runciman]. But in fact there is no contest. A single author cannot speak with the high authority of a panel of experts, but he may succeed in giving to his work an integrated and even epical quality that no composite volume can achieve... I believe the supreme duty of the historian is to write history, that is to say, to attempt to record in one sweeping sequence the greater events and movements that have swayed the destinies of man. The writer rash enough to make the attempt should not be criticised for this ambition, however much he may deserve censure for the inadequacy of his equipment of the inanity of his results."

His vision of history is one of bold, ambitious, sweeping narrative. So to tie together events as chronologically sparse as the fall of the Western Roman Empire and the Fourth Crusade was in keeping with his attitude to history and personal dramatics.

The other thing is that although Runciman is most known as the guy who wrote that history of the crusades, he was a Byzantinist; someone who studies the Byzantine Empire. He was enormously interested in the Byzantine Empire. I don't think it would be an exaggeration to say that he was in love with it. In his work he tends to be sympathetic to the Byzantine view of historical events, tends to treat Byzantine sources with more credibility than they usually deserve, and it may even be fair to describe Runciman as the last Byzantine historian. The Byzantine sources look down on the crusaders as simple brutes; good at war but not much else, and certainly not as culturally advanced as the great heirs of Rome, and Runciman agreed with that view wholeheartedly.

So his view of history, and his "supreme duty" as a historian, was to weave together the great narratives of the past. Runciman liked to tie together chronologically distant events and make them work together. He sees the crusades as a continuation of barbarian campaigns in late antiquity in large part because those are both things inflicted on the Roman Empire by western peoples, and his view of history is one that inclines him to connect them, leading him to a grand hyperbolic statement that appeals to his sense of flair. And he stood by that statement because his view was fundamentally similar to that of the Byzantine elite, which did view the western crusaders with contempt as simple barbarian peoples. On a more emotional level, he considered the Fourth Crusade an immense crime because it had ransacked and almost destroyed the love of his life. So yeah, it's a hyperbolic statement that historians don't give much credence to.