If those yields are cumulative then that results in a 93% or lower yield. That's not a disaster, but it's not great either, and quite a bit lower than typical packaging yields which are in the high nineties. In addition, a packaging failure may not only destroys the main die but all 4 HBM stacks as well. (I wonder if they package everything in one go, or if it you have separate phases, with continuity tests in between.)
The description of the test vehicle notes that it has 100k micro-pillars with no designed-in redundancy, which may make the 95% figure pessimistic if actual products add some redundancy.
All the descriptions of AMD's HBM offering do not have Globalfoundries as the interposer supplier, which may make the figures weaker approximations for what to expect with the upcoming product.
There is always the possibility of some unheralded OEM or niche-market salvage SKU with a weird salvage configuration if it comes to making some money from discards.
The test interposer's reticle-size dimension is a touch more modest that some of the research projects had projected for an interposer implementation at 26x32mm, but it's quibbling over a few tens of mm2.
AMD's artistic renditions of the HBM package seem somewhat disconnected with other images they provide, and what a known good stacked die from Hynix has been described as.
AMD round down a touch for its memory dimensions, with a 5x7mm module listed by Hynix as 5.48x7.29mm +/-25um.
http://www.hotchips.org/wp-content/uploads/hc_archives/hc26/HC26-11-day1-epub/HC26.11-3-Technology-epub/HC26.11.310-HBM-Bandwidth-Kim-Hynix-Hot Chips HBM 2014 v7.pdf
http://images.anandtech.com/doci/9266/HBM_12_FormFactor.jpg
The picture AMD provides of a mounted HBM module might not be what was being drawn, maybe a test device? Hynix has the PHY laid out the length of the module, and this dimension is usually kept matched by the side of the main SOC and very close by, as seen by Kalahari, other proposed HBM designs, or the photograph in AMD's slide. Not so in AMD's diagram, which has them hanging half-past the corners.
That pictured module isn't near a corner, assuming the dark expanse in the background is a main die. That module may be near another stack, but not consistent with how AMD's slide or Kalahari are drawn.
The interposer may be significantly smaller than Kalahari. Those dimensions, absent knowledge of how much room must be kept free for routing or other purposes, leave room to the imagination, which might be AMD's intent.