I've always found this claim to be odd. Not to start an argument, or derail the thread, but isn't BIAB a form of steeping, yet we call it mashing?
I could be totally wrong here, but I don't get why people think the grains know they're in 154° F water because they're being steeped or if they're in 154° F water because they're being mashed. Either they're sitting in hot water or they aren't. Back when I was extract brewing, I got super bugged by someone telling me I needed to mash a Munich malt addition and not steep it. I thought, "Maybe I just won't tell the grains they're being steeped for an hour at 154° F, and they'll think they're being mashed...."
Are people assuming that steeping means the grains are clumped together in a tight muslin sack, and mashed grains are more separated and free-floating? That's about the only difference I can think of between the two. -or is the assumption that people don't mill their steeping grains? I feel so dumb for not understanding this.