You were told wrong.
For the Optimus M to operate on LTE, it would need LTE chips installed inside to trasmit/receive on LTE frequencies. There's currently only 5 LTE phones in the entire country: Metro has 2 (Samsung Craft, Samsung Indulge) and Verizon has 3 (and their version of the LG Optimus is not one of them).
Metro and Verizon have different LTE frequencies so they are not compatible on each other's networks.
Our phone is based on the LG Optimus One, which in the name it's called the rest of the world. Here in the U.S. and Canada, it tends to have a name or letter associated with the carrier (M=Metro, T=T-Mobile, A=AT&T, C=Cricket, U=U.S. Cellular, S=Sprint, V=Virgin, Vortex=Verizon, etc.) They all operate on CDMA or GSM -- -- in other words,
nothing higher than "3G". Even Verizon's version is CDMA only -- and Verizon is the other LTE network already running in the USA.
This phone was released last fall (2010) to the world and the design is almost a year old. Metro was the first U.S. carrier to get it, in November.
At that time Metro also had the first LTE phone in the entire world -- the Samsung Craft, but it's not an Android and not even a true smartphone. It was a "feature" phone slapped together by Samsung for Metro to launch their LTE network and beat Verizon to have first LTE network and phone in the USA.
Only in Feb 2011, did Metro release their first Android LTE phone, the Samsung Indulge -- also beating Verizon's first LTE phone (which was an Android). Metro had two LTE phones out before Verizon had any.
But both Metro's LTE phones were $400 when released and both are still $300 even now. Whereas, the Optimus M is now $99.

IF it could send/receive LTE, believe me it would be $300 too!
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Metro's 3rd LTE phone (and 2nd LTE Android) is currently scheduled to be released in mid-October. It will be an LG phone, but not the Optimus or even based on the Optimus. It will be the
LG Esteem (code-name Bryce) which is based on one of Verizon's LTE phones, the LG Revolution. Has a 4.3" screen, 1GHz CPU, dual cameras, LED flash...