I wonder if the information provided by Humble on that book is accurate in the first place, even the official O’Reilly site still refers to the first, 2014, edition.
Others seem like they could be dated, as well, I spotted HTML and CSS 2nd Ed being a 2012 book, and Java 2nd Ed from 2005.
OTOH, Go and Kotlin are from 2019, Android 2017, Python 2016.
On the whole, this might not be as big of a problem as it may first appear, since all books are aimed at beginners, and the all-inclusive top tier is only 15 Euros.
I’m not sure about sales, however many of these (e)books seem to be retailing for $40 each on Amazon. Going with Humble you get three different DRM-Free formats (PDF + Mobi and ePub), instead of just DRM-Encumbered Kindle.
That said, Humble only offers short PDF previews and that gives no indication what quality the eBook conversions might be.
All in all it might be worth checking out the Amazon reviews for the books one might be interested in before pulling the trigger.