They are making the argument that total perceived time would still increase logarithmically as absolute time increases, even under the model discussed. (And doesn't, say, approach some fixed sum.)

Hence there's still a point to trying to live longer, even if the value of each additional time period decreases.

Anyway, that's if you agree that model! There's a couple of other things I'd consider without even examining it's core assumptions: memory fades over time - time earlier in my life might not contribute as much to my perceived time, as recent time does; what if you spend your early life just getting to the place you want to be (resources, self-development etc)? Extra time in that place could be very valuable!