The idea is that the old php syntax "function foo ()" remains unchanged [1] so developers can keep searching for "function foo". Removing "function" as keyword is a different topic and is discussed in a separate php thread [2].
The thing you mention ("float function add(){}") keeps the option to search for "function add", but I am unsure why this is not taken into account in this proposal.
Anyhow, I am pretty happy this finally made it to the php language.
[1] https://wiki.php.net/rfc/return_types#position_of_type_decla...