As long as the oven only needs the output from 3/8" then there is no issue with this at all. I would think that that 3/8" could supply any home oven (that's still a lot of gas) so I don't think there is an issue at all. If the manufacturer agrees then there is no reason for you to worry about it.
Frankly the installer probably installed the shut-off that he normally installs for ovens. Most ovens are fed with 3/8" or 1/4" so he might not even have had this on him at the time.
This is akin to having a 1" water line going to a bathroom shower and a plumber using a 3/4" shutoff for it. There are almost no shower valves that can even take 3/4" of water and certainly not 1". Lazy I don't know. If your shower has inputs of 1/2" or less it doesn't matter. Just like if your oven can only take in the output of X (which may be less than a 1/4" feed) then it doesn't matter. It only matters if he installed it right.