The fact it requires a specific version is fine, and makes sense, and you can make it in big red letters for people to not miss and people will still miss it.
Something to consider next time: bumping the minor version would also have made sense here or, if possible (I don't know what new code has caused this requirement), have two seperate releases. One for the patch fixes, and then a minor version upgrade for the new PHP version (which is still old so you've got the ability to be backwards compatible to a degree).
Personally, I saw this as a patch upgrade and assumed it would be upgrade without fault. Every other SMF patch upgrade has, until this one.