What they were saying is specifically that arguing that someone who takes a life should be punished in a life ending way because of the severity of taking a life is undermining that argument's own central claim of the severity of taking a life by at the same time arguing for ending a life.

Your analogies are flawed for two reasons:

1. there are other justifications for punishments that may (or may not) be valid. One argument may be that it is preventative, for example.

2. in neither example would most people argue that we're imposing a particularly extreme sentence because the action was particularly extreme, and this is central to the argument made by graderjs.

In the case of the theft example, if anything we'd tend to argue a fine is not a very serious punishment. In the case of kidnap, we argue for serious sentences because of the seriousness of the crime, but a prison sentence would not be seen by most people as equally cruel as the uncertainty and fear and trauma imposed on both the victim and their loved ones, and most people would argue that the prison sentence is not just a response to the seriousness of the crime committed.

Some people might nevertheless find it hypocritical to argue for prison sentence on the basis of the seriousness of kidnapping, and its their right. But that in itself does not make a prison sentence unjustifiable even if you agree with them, because you may have many other arguments, such as e.g. risk of reoffending, or signal effect to others.

The point is in other words not that a given punishment can't be justified, but that if you choose to argue that something is particularly severe and then in the same breath argues for inflicting that same thing on someone because of that perceived severity, you're undermining that argument.