I think people are talking about two similar but very different problems.
1. People from European countries who leave their country to go fight for ISIS/extremists that then attempt to return home.
2. People who are radicalized online via social media, Twitter, propaganda, etc.
The terrorists in the Paris/Brussels attacks were people who themselves left the country they were from in Europe, went to fight in Syria then returned home, or were close friends of people who did. While there ARE people who left the US to do this, the US has what is regarded as a pretty good grip on who those people are. Part of the problem in Europe was the sheer quantity. The other part of it is you can make your way back to a Belgium from Syria fairly under-the-radar, especially as a citizen of a European country. You have to fly commercially to get back to the US - not under-the-radar.
That being said, our intelligence only matters so much. If Belgium lost track of one of their citizens who went to Syria and came back and then that person then flies to the US - we're also relying on other countries to be doing well with intelligence. Keep in mind - on the no fly list, there's over 80,000 people and only around 1,000 of them are Americans.
Now, onto problem #2 - that is a much harder problem to control. ISIS has done a very good job in branding itself, promoting itself, putting out propaganda, etc. It's very influential and very modern in its abilities. This is obviously much harder to track and pin down people. This is the biggest problem for the U.S. at this time.
Looking at the Orlando attack, it's hard (at least at this point) to figure the details out. He mentioned three different groups - all of which do not get along and are actually enemies of each other.
As for your problem number 2, it is hard to defeat, because even if ISIS is 'defeated' on the battlefield, it is an idea as well. Hard to defeat an idea that is marketed pretty well through new age social media, madrass, etc
As for the Orlando attacker and the issues you bring up, understand that what you say is a pretty common issue in the ME. Many a time Muslims/middle easterners will end up supporting groups that they are an enemy of due to 1) Religion 2) Lack of other options.
Perfect examples right now would include
1) Residents of Raqqa that are not in ISIS, possible have skirmished with them, will fight along side them when the Kurds start getting closer to Raqqa. Why? Because they view it as their home being attacked by outsiders and they dislike the Kurds more. Throw in loyalty to religion as well.
2) Christians and Sunni are fighting along side Hezbollah versus ISIS. Three groups that are at the very least wary of each other have a bigger common enemy.
I believe in your example of the Orlando shooter he pledged allegiance to ISIS and support for Hezbollah. A contradiction indeed. That said, they do have the ulitmate enemy in their eyes being Israel.
Just a lot of different sides, a lot of sworn enemies, that often can find a larger common enemy. Just being on the ground over there one can tell a few different things. 1) Most people just want to live in peace/stability and get on with their lives 2) The confused geo political/religous wars there make the entire ME/Western Asia a much more complicated/convoluted place than people realize.