I read that too and had a similar epiphany orange. I'm very curvy and one of the most flattering dresses I own has thin horizontal stripes. I steer clear of bigger stripes, but that might just be because I don't suit lots of contrast in my colours.
I figured this must be myth as pretty much every shop had horizontal stripes in the windows at the mo! Am considering making my first Boden purchase & didn't want to make a mistake ordering stripes in the wrong direction!
Think it is more down to colourscheme used, how much, where you wear them and the width, I suppose if you follow the usual rules about emphasising your good points and disguising the less than perfect, you cant go far wrong. Balance of overall outfit is key to it all I suspect, so checking outfit in a full length mirror is probably a wise move!