It's difficult to explain why Corbyn would be so unpopular otherwise.
There's the whole career long siding with the UK's enemies thing? Hamas, Hezbollah, the IRA, Iran, Russia .... Rights and wrongs of these issues aside, he has an instinctive bias against US and UK interests. I genuinely think he'd be a security risk if in the top job. The Skirpal poisoning is a case in point. Corbyn wanted to send the guys who did it a sample of the weapon they'd used! I foresee Trump-like briefing against his security service officials, should Labour succeed in getting in.
Re. the case shown in the clip I posted above. I don't think it's just worth me rehashing the same points over and over with increasing animosity. But 1400 died in that attack. The Russian misinformation campaign in its wake was huge. Corbyn plays a role in this strategy (unintentional, I'll grant) in helping launder the misinformation, giving it a veneer of credibility. You'd be outraged if someone from the govt benches was doing the same with say, the Saudis in Yemen. It's not a neutral fairhanded position, it's one taken as a counter to the interests of the UK.