SEJeff obviously isn't saying that Trump support and IRA support are exactly equivalent. He's saying they're both subgroups in a larger population that outsiders stereotype as representing the entire population, to the disgust of many of the people stereotyped. It's an excellent analogy because it references a situation that's intimately familiar to the person he's speaking to.
38.6% of Americans state that they would vote for Donald Trump over Hilary Clinton while 49% state that they would vote for Clinton over Trump.
There are only several hundred members of the IRA today (out of 1.8 million people in Northern Ireland) and even fewer of them participate in terrorism.
And it's not a logical fallacy. The cab driver clearly didn't mean all Americans support Donald Trump. Presumably he knows there are other candidates so thats at least a few people who don't support Trump. He meant a lot of Americans do support Trump and clearly they do.
[1] http://www.realclearpolitics.com/epolls/2016/president/us/ge...
[2] https://en.wikipedia.org/wiki/Real_Irish_Republican_Army
Why are you willing to accept that the cab driver was speaking figuratively, but not that SEJeff was?
Then why was he surprised to meet an American who didn't? You're really reaching.
It's hardly reasonable to compare being a member of a terrorist organization to supporting the leading candidate of one of two major parties.
Consider just the sheer effort of participating in terrorism is far greater than voting.