Replacing rot in wooden porch column
In a word: Yes. It is perfectly acceptable to replace a portion of a paint grade column so long as you do it correctly. The technique depicted in your video-link, however, is not up to snuff. I scoured youtube for a video to no avail but here's a image of what it should look like:


Basically you need to support the load with prop studs or elephant jacks, remove the post, then cut out any and all rotted areas (if the rot extends into the turned area you're in trouble, this requires a different fix). Using a skill saw or exceptionally sharp hand saw, form the joint on the post end then lay it on top of a piece of identically dimensioned square stock and transfer the lines with a sharp pencil or better yet a scribing knife. Cut the mating joint on the extension, check the fit, then cut the extension to length. Join the two members with construction adhesive and lags as shown and then bondo any defects and sand smooth. Re-install the post and enjoy. Oh and make sure to butter the fresh end with either paint or oil or some sort of sealant first.