I think the photo is a bit misleading - even the 33.9mm diameter post-pump is only 580mm long. The smaller diameter ones are only 350mm long. So it's more of a compact high pressure pump than a proper floor pump.
At $25 I expect it to be cheap and awful. A decent aluminium seatpost costs that much without the pump. I would be concerned that it would wear out, and parts would not be available. I have seen other types of seatpost pumps and the only one that seemed to work was a $100+ one built into the very long seatpost of a folding bike. The one I saw in a mountain bike combined the irritation of having a bike seat attached to one end of a bike pump with the poor build quality of a gimmick. From memory that one broke - and having your seatpost break is not a fun experience.
They don't mention the weight - it's quite probably heavier than most combinations of seatpost plus pump.
One problem with having the pump inside the seatpost is that you don't see it. So you're unlikely to maintain it, or even check it on a regular basis. One attached to the frame or in your toolkit is more visible. And unless there's a scale engraved into it, every time you pull it out you're going to have to readjust the height of your seat.
I suggest looking at other sub-$25 bike pumps and thinking carefully about how cheap the pump part has to be for them to add a seatpost to the mechanism.