note - this is without having watched the video:

The technical meaning of "I don't understand" is "I do not comprehend the processes by which this thing (in this case death-ism) occurs." It's an invitation to explanation, for the sake of increasing your own understanding.

However, in rhetoric, the meaning often alters to "I am so strongly opposed to this viewpoint that the very fact that anyone considers it valid dismays me. The viewpoint is in fact so ridiculous that I doubt anyone who subscribes to it can give a coherent explanation as to why." It's also an invitation to explanation, but in this case the purpose of the explanation is to force the opposing argument to articulate what you perceive is a hopelessly weak point which which will be shown to be self-evidently false via the very process of articulation.

Kawoomba thinks that using "I don't understand" by the rhetoric definition is inherently insincere, and he assumes that you are using it in the rhetoric definition as well. (For the record, I am neutral concerning whether or not using the phrase rhetorically is inherently insincere.)

yet I just can't seem to wrap my head around how this viewpoint even exists.

Were you, then, truly using the technical definition? Do you actually feel you do not understand the mental processes and lines of thought by which people decide that immortality is undesirable? If so, it's worth discussing because it's generally bad form to give a strong dismissal a viewpoint that you know you don't fully understand.