Yep, it's mostly bollocks
For those who don't know - AIS basically broadcasts in plain text a vessel's position, heading, speed, ID and a couple of other ancillary bits of stuff. You typically view it on a chart plotter when navigating busy traffic lanes so that the collision-avoidance software can put flashing red circles on the chart to warn you where your collision risks are. Of course you can capture the information and use it for other stuff if you really want to. It goes out at low power on VHF radio.
It was EXTREMELY useful the last time I sailed over to the Channel Isles in a small yacht because the kit is cheap and simple to install and when you are on a 35' long vessel that may not be easily visible from the bridge of the supertanker 10 miles away, it adds an extra layer of safety. We could plainly see the vessels on collision course with us altering course a degree or two to avoid us (as we were under sail, not power). I have a man-overboard AIS transmitter sewn into my lifejacket just in case it's needed - very helpful to lifeboat crew and the search and rescue people in an emergency.
Once when the skipper ran us aground in the Thames Estuary an incoming bulk carrier saw that we were located not moving over a sandbar and called the coastguard (embarrassingly, I must say) on our behalf. Te be fair it was blowing a gale and the sea was rough, if we hadn't got off quickly the boat could easily have broken up, the coastguard were quite a lot more concerned than we were, as we'd actually got 30m off the sandbar but chucked the anchor over to stop us getting aground again until the tide had risen a metre or so.
Yes, any muppet could fake those signals, though it's hard to see what they would gain from it apart having a bit of a laugh with the coastguard - or maybe to confuse those on the bridge of vessels receiving the information. But you augment what you can see on AIS with the mk 1 eyeball and binoculars, you don't RELY on it, unlike GPS.
AIS is a cheap and easy aid to safety, as I see it. Not a lot more. Oh, and very, very handy in fog when you need all the help you can get.