I find this sentence confusing, kind of like 'compatibility' is being used as a definition for compatibility. Maybe removing the second occurrence would make this more clear.

Also, what is being described here fits the definition for the term MAY or OPTIONAL. It might be a good idea to try and work in either of these terms.

As a quick stab: Note that Implementing Libraries MAY use PHP's serialize()/unserialize() functions internally but are not required to do so. Compatibility with them is simply used as a baseline for acceptable object values.