Here's an explaination of this. The tip is true. It's not even a grief or a hack of any sort. The way the game works is it checks your GPS position every minute, then every 4 minutes it updates your distance traveled into your eggs. Now, if you have a bad GPS signal or are in a building that kinda makes your GPS drift, it will make your avatar move a little bit here or a little there. This effectively makes your phone think (inside the game) that it has moved too that far. Maybe he walks a block away and comes back. IF and ONLY IF he's gone away when that minute check happens then you'll get credit for walking that far and back, IF he comes back at that right moment too. This is literally adding 100 meters or 10 to 20 meters of distance at a time. Maybe. BUT, if you add that up all day while sitting at a desk working in a big building with bad GPS signal, with your game on and your phone charging you'll get a few kilometers in. I learned that in a thunderstorm my phone does this. It happened while I was doing things inside the game and not moving from my desk and my Egg that was .5 K from hatching hatched.
THE BAD THING HERE... If you are walking around a block, and the GPS pings slight before the corner and again slightly after, you only get credit for the straight line between those two points. So, it's good and bad. This also means, the best way to hatch an egg is to walk straight lines. A big circle will only get your straight lines throughout the circle cutting off large portions of the circle. This too I test, there is a 1/4 mile circle in a park (not a track) and I get signficantly less on my eggs than I get on my Google Fit app. So, take the good with the bad.