No, you completely misunderstand the phrase "He actively supported their campaign of bombing in mainland Britain, and argued against those who though otherwise" @ImWell typed.
They clearly meant Corbyn was in favour of the Provisional Sinn Féin and Provisional IRA campaign of terrorism. In other words, he was in favour of it. This is not illegal itself.
See Labour's David Winnick's talking about how he argued AGAINST Corbyn that the Provos were NOT justified in their terrorism as well. It's just not an illegal act to argue you believe PSF/PIRA were justified even though they were not.
Abbott and McDonnell had the same view as Corbyn - they supported PSF/PIRA terrorism against the British State and the British people.
Had Corbyn's sympathising went as far as to actively aid and enable the Provos to carry out a bombing campaign....he'd have broken the law and been arrested.
As for how the Security Forces regarded Corbyn, this is common knowledge top in Northern Ireland, Westminster and Dublin Castle:
"I used to look at him when he used to visit Belfast in the 1980s, when he was running around with guys who had done some hortible things, and think: "who is this guy? What is he thinking coming here?" I think our colleagues in England would have been more concerned with his antics. They would have panicked at some of what he got up to."
And:
"He (Corbyn) was someone who was bringing these people (PSF/PIRA) just to put pressure on the government, as opposed to saying, "You need to get these people (PSF/PIRA) to stop the violence and then we can so business...that these people need to make a declaration of intent to say we are going to stop, and that wasn't coming from Jeremy Corbyn."
Corbyn also supported the Troops Out Movement which was a Republican/PSF/PIRA front group. The aims of this organisation was also delegitimised by the GFA. Troops Out Movement was really a pro-IRA movement aimed at convincing enough gullible, naive uninformed British people in Great Britain to pressure the UK Government to withdraw the British Army completely. Such a demand would NOT have led to peace, but to all out civil war in Northern Ireland. It's other aim of "self-determination for the Irish people as a whole" also clashes with the GFA which does NOT give this as it recognises Northern Ireland's people are the only ones who can make decisions for Northern Ireland's future.
Some Provos have since admitted they "just couldn't crack that nut" in turning the British people against the British Army and Ulster's British citizens.
Whichever way you turn, Corbyn, Abbott and McDonnell constantly supported the aims, agendas and methods of the Republican Movement aka Provisional Sinn Féin and Provisional IRA aka The Provos.
Supporting such dangerous terrorist organisations in this manner isn't illegal. But aiding and abetting terrorism is which is something they didn't do.
So, no, you continue to misrepresent what @ImWell actually said as you're not doing well in the debate at all.
One thing you will never find from Corbyn is a statement condemning PSF/PIRA violence UNEQUIVOCALLY. A genuine, reasonable and honest person would condemn any terrorism in strictly unequivocal terms when asked to do so. Corbyn never, ever dies.
I can condemn PSF/PIRA unequivocally in the same way I can condemn the PUP/UVF unequivocally. I do NOT under any circumstances need to qualify condemnation of PSF/PIRA with that of PUP/UVF - it's not necessary, but is something a supporter of each terrorist group actually does. 🤔