North Korea wouldn't use an ICBM to attack Guam since it's only 3500km away. They would use an Intermediate range ballistic missile for it. ICBM's are missiles with an flight range of at least 5500km.
Both those links are about the Ground-based Midcourse Defense system which is designed to protect the US mainland and Canada from ICBM's by taking them out in the midcourse phase. It has nothing to do with the protection of Guam.
The THAAD, Aegis and Patriot systems are more local systems that are designed to take out incoming missiles in the terminal phase and these are the systems that would be intercepting missiles fired towards Guam, Japan or South Korea.
All 3 of these systems have been tested a lot and successfully too and are now deployed in various parts of the world.
When it comes to taking out missiles there is never a 100% accuracy to it. There is always something that could go wrong and even if they fired just 1 missile there is always a small probability that it could get through.
If you want to read more about the US and South Korean defenses against an missile attack from North Korea I can recommend this PDF
https://www.38north.org/wp-content/uploads/pdf/2016-03-10_THAAD-What-It-Can-and-Cant-Do.pdf