This seems to fit a different context than the sort of argument being made by Linster. The primary objection to Linster is not a disagreement on what is possible but a disagreement on what is good, moral or just. Your argument is an argument primarily about possibility based on the physical constraints of our universe. I don't think many people here will disagree with your assessment.

If for example it turned out that we could bend space and violate conservation of energy using some advanced technology to make the limitations you point out obsolete Linster's argument seems to be that immortality would still be a very bad thing.