Biden, as stated above, will want to see the Good Friday agreement honoured. The Americans had a considerable input into the agreement. It allows Loyalists to see themselves as British and Republicans to see themselves as Irish. And it has been reasonably successful.
So his comment of “ I’m Irish” may have simply been a setting of the scene re that.
If something, like a no deal Brexit, happens to endanger the current soft border, then he has the option of saying “no trade deal” to the UK. He might do that, but he is portrayed as a pragmatist rather than some Trump like figure.
The IRA sympathiser bit, at this point in 2020, is just rubbish. And that’s where we are 2020, not 1992 or 1994. US views on terrorism changed dramatically after 9/11.
What some people in the UK don’t seem to get is that the consequence of leaving the EU is that we move from being a major player in a 500 million people union (with the great advantage re the US as being English speakers and to an extent able to voice their concerns) to a much more marginal player, with a population around 15 per cent of the remaining EU.
The UK is therefore much less important to the US than it was. Not unimportant , but less important.