You do know this is marketing bullshit don't you?
There is a degree of this. I've ridden steel hardtails that are harsh, and alu hardtails that are fairly springy. Suspect it's as much down to frame design as material.
Prince Alberts are stiff and heavy. Not very forgiving. I had one for a while and didn't like it one bit.
On the other hand, I had a Kinesis Malight XC120 and it was the polar opposite, superb comfortable and forgiving ride, probably one of the best bikes I ever had.
Listen to these guys, refreshing that some people on here actually know what they're talking about!
It's as much, if not more down to frame design than material used, whether or not a frame is stiff or "forgiving". You can make a stiff and unforgiving frame out of titanium, and a very forgiving one out of Carbon or ally if you know what you're doing! I've ridden a number of stiff and unforgiving steel frames (Cotic BFe, Dialled Bikes PA, a number of Dirt Jump style bikes) and I've ridden a few very forgiving ally frames. OK, all things being equal, a steel framed bike will be a bit more forgiving and resilient than a similar ally frame, but often not all things are equal, remember that.
All the above said, I still really like steel as a frame material when it's done properly. The Cotic Soul is one example that makes the most out of the material, another is/was the Genesis Altitude/Latitude.