Yes, it was technically theft. Officers found the vehicle at his house shortly after it was reported missing. Not really an opportunity to return it. If all laws were prosecuted to the exact letter of the law we would not need juries. The DA will prosecute any case that they feel has enough evidence to secure a conviction, as this case had. It is the juries that determine if prosecution in each case is "just".

Steal a car to rush somebody to a hospital then return it after and the letter of the law will convict you as a felon. It is the jury that could decide, as representatives of the community, that you should not be a felon for saving somebody's life even if you technically stole a car.

In most states the jury instructions are to make their judgement based solely on the evidence presented to them. These instructions make a jury a cog in the prosecution machine. The purpose of a jury is a final check on the government's execution of the law by members of the community.

If they had previously allowed him to take the vehicle home, then I would presume he probably was planning to bring it back. Once you explicitly allow a person to do something, it becomes harder to claim harm when they do it again.