Look at a cable as a pipe and a video signal as the water being pushed through it.
If a pipe is smaller than the volume of water being pushed through it, like a component cable is (in its inability to produce a high quality signal), it will slow down and constrict the amount of water being pushed through it. A component cable plugged between a Bluray player and a TV would do the same...condense the signal to what it is able to produce, and that would be the max signal your tv receives.
Now if you enlarged that pipe, ie plugged in a hdmi cable, the maximum amount of water would be allowed to flow through to the final destination. Doing that did not change the original amount of water being received did it? No more than a cable will take alter the source it receives from your Bluray player.
Nor will it change the circuitry on your tv and make it better. Imagine...why would we ever upgrade our older plasma/LCD screens that had slower refresh ratios...we could just by a cable and magically improve our old hardware.
It is a classic case of a moron salesman talking out his but.
The once the video signal is received by your tv, that is where your refresh ratios (60/120hz) come into play...that is after the signal has already passed through the cable! The cable has nothing to do with it. Refresh rates (hz) are determined by the circuitry of your tv...NOT a cable.
That being said, there are some ther things to consider. Depending on your viewing needs, you may nt need a tv that refreshes at a rate higher than 60 hz. If you are just mostly watching cable/satellite you are not going to receive anything that would benefit from that faster refresh rate. And by the way, do not be fooled by sat/cable providers - you are not getting 1080p signals from them. It is certainly better than it used to be, but it is not even close to a full 1080p.
If you are a big movie buff and watch a lot of movies or play video games a lot then you may benefit from a higher screen refresh rate. Movies are filmed at 24 frames per second. If your tv is only capable of 60 hz, it may not show a movie in its full splendor when compared to a tv refreshing at 120 hz.
Remember, an awesome video signal comes from a combination of things, not just your tv. To have the best available signal out there today you not only have to have the best available tv, but also the best available Bluray player playing the best available bluray disc and have them connected with the proper cable ie high speed hdmi.
Not all TVs are created equal, not all Bluray players are created equal, and not all Bluray discs are created equal. The only thing that doesn't affect things is your hdmi cable...a cable is a cable and always will be just a cable.
With all that said...I went from a 42" 1080 p panasonic plasma that had a 120 hz refresh rate to an epson 1080p projector that only has a 60 hz refresh rate...and I love it a thousand times more, despite it being a technology from a few years ago...why? Because it is displaying my movies at a glorious 106" and able to show them up to over 120 if I had space. Movies still look incredible...Avatar was more fantastic on the slower projector than on the faster plasma! I noticed no choppiness in the fast scenes....and that is why this whole hz thing is still something debated.
But in answer to the original question...no...the sales guy lied to you. If your manual says 60hz than it is 60 hz. Boxes lie sometimes, salesmen lie more...unless you get one of the few good ones like I used to be.