I checked with the Corridor Manager of this section of the A.T., and he explained to me that the yellow markers you saw on the short A.T. section between the New York-New Jersey state line and the State Line Trail are actually used to designate the A.T. corridor boundary. They do not represent a trail; rather, they are used to mark the boundary between the A.T. and adjacent private lands. In the case at hand, the A.T. runs very close to the corridor boundary, so some of the yellow blazes are visible from the A.T.
Really enjoyed this hike on a beautiful late August day. Took the public transport Bus #197 to and back, worked out just fine. I took the 7:30am bus from Port Authority which arrived at location around 9:30am, allowing for me to take the bus back at 4:20pm. Surprisingly rugged terrain (which included use of hands at several spots), thick vegetation and occasional lack of signs (the AT is pretty low-key and sometimes just seems to "vanish" in the bush) made for a hike that was tougher than expected, but also way more fun. I had a print-out of the description which proved very useful when in doubt.
The only issue I had with the description is that it doesn't mention that the AT meets a yellow-blazed trail shortly after the state line, but BEFORE meeting the blue-blazed State Line Trail. Some, like me, might mistake this for the yellow trail mentioned further on in the description, which led me to believe that I had missed the turn onto the blue State Line Trail. Just ignore that first yellow trail as well and keep going on the AT and the blue trail will eventually show up.
It took me about 5h30 to do the trail, which included at least 1h of breaks and enjoying the views, the track is definitely doable within 5h for a steady hiker. I also got to relax at Greenwood Lake and dip my feet in the water once done, that was a bonus. Highly recommend the trail for motivated hikers looking for a day trip.
Yes, this hike is a "point-to-point" hike, not a loop hike. This is made clear in the description (to see the full hike description, including the introductory material, you should always click on the "printer-friendly version" to view the hike). However, as is also made clear in the hike description, it is possible to return to the start of the hike via NJ Transit's #197 bus. The bus does not run all that frequently (it generally runs once every two hours), but with careful planning, you can use it to return to your car (or, even better, you can start by taking the bus to the other end of the hike and then hiking back to your car).