There's two problematic things here, "natural attack" vs. "natural weapon", and "weapon" vs. "manufactured weapon".
Natural attacks are light weapons
The game itself doesn't always use the correct terms, but a natural attack is an attack with a natural weapon*. A natural weapon, as the name implies, is a weapon**, and thus for most instances where the rules talk about weapon or attacks with wepaons, a natural weapon or the natural attack made with it are included.
There are some exceptions:
• Everything in the equipment rules and magic item rules is only about manufactured weapons. Natural weapons count as light weapons for stuff like Weapon Finesse, but not for general rules.
• Everything that mentions main hand/primary hand or off-hand only affects manufactured weapons and unarmed strikes. Example: Double Slices doesn't help secondary natural attacks, and you can't TWF with natural weapons.
• The rules on combining naturla attacks with other attacks, found on pg. 182 of the CRB, use "melee weapon" to mean "manufactured weapon".
• The Natural Attacks section of the Universal Monster ruels (b1 pg. 302f.) are likewise using "weapon" when the ymean "manufactured weapon".
For the question at hand, the term "melee weapon" in the CDG rules means "manufactured melee weapon, natural weapon, or unarmed strike".
*) Yes, that means the entire section in the Beastiary about natural attacks is erroneous.
**) Always melee, by the way. Even if a creature can shoot some body part, e.g. a Manticore's spikes, those are not classified as natural wepaons and don't use the respective rules like strength bonus or the primary/secondary differentiation. They do count as ranged weapons, though, both for general rules like that you use dex on the attack roll, and for individual options like Precise Shot.
Unless the slam is able to be done at ranged it is a melee attack. So, unless the natural attack is at ranged it is considered to be a melee weapon.
Welcome to the rules forum explains why strict RAW is insane with Piazo's spaghetti rules episode 293,626.
Welcome to the rules forum explains why strict RAW is insane with Piazo's spaghetti rules episode 293,626.
On the contrary, the RAW is perfectly clear if taken literally, and for the issue at hand, the RAW is perfectly fine. The issue is that there are other parts of the rules where going very strict RAW breaks them.
Unless the slam is able to be done at ranged it is a melee attack. So, unless the natural attack is at ranged it is considered to be a melee weapon.
But making a melee attack does not equate using a melee weapon. Delivering a touch spell is a melee attack, and yet it's not done with a melee weapon.