I'm speculating a bit here, but I suspect that answer reflects a practically risk-averse read of the same case, for two reasons. One is that, as a district court opinion, Bridgeman isn't _necessarily_ binding federal precedent. (I don't want to overstate that point, because Bridgeman is well established and has been found persuasive by other courts. Still, given that this site appears to distribute the resulting ebooks itself, I assume their appetite for risk here is pretty small.)
Even following Bridgeman though, there is a teeny bit of analysis that you have to do to determine that the reproduction is a faithful photographic copy of the original. In most cases that's straightforward, but again, if you really want to limit risk and as in this case your application doesn't require a particular image, it might be worth cutting out that analysis altogether and going with media that is simply outside of the range of copyright terms.
Even following Bridgeman though, there is a teeny bit of analysis that you have to do to determine that the reproduction is a faithful photographic copy of the original. In most cases that's straightforward, but again, if you really want to limit risk and as in this case your application doesn't require a particular image, it might be worth cutting out that analysis altogether and going with media that is simply outside of the range of copyright terms.