I'm not sure where to start, to please excuse my regurgitation of information below:
First off, there is NO working build of 2.3 Gingerbread for the Ally. There are two projects, but both have been put on hold due to problems that cannot be fixed as of right now.
That link you posted about upgrading to 2.3 is a complete troll thread, and more than likely an attempt to make people brick their phones, forever. The files contained in that post are 3.0 Honeycomb for the Nexus S.
As for the video, yes, that is legit. Velocity is one the few ROMs currently available for the LG Ally. I suggest that you read more on rooting and installing ROMs on the Ally (And keep it within the forums for the most accurate information.)
For a fast, stable, badass ROM, check out either Velocity 1.2 or StormDroid 1.2, both are very fast, stable, and awesome.
If you still are completely left in the dark,
come hang out in the IRC channel and those guys can help you out.