The Unchained Eidolon's bipedal base form lists these starting statistics:
Size Medium; Speed: 30 ft.; AC: +2 natural armor;
Saves: Fort (good), Ref (poor), Will (good);
Attack: 2 claws (1d4)
However, the subtypes list additional evolutions for the bipedal form, like the Elemental:
Biped (limbs [arms], limbs [legs], slam).
The question is: an elemental biped eidolon would have both the claws and slam evolutions, or just the slam?
I don't see if it is specifically mentioned anywhere but it seems that each pre-made form is replacing the base evolutions if its more thematic. Since i noticed ancestor is suppose to mimic player character races and doesn't mention claws, where as the devil and demon mention claws.
Another way to look at it is that it mentions arms and legs in addition to the slam. So if you still retained claws, that means you would also have 4 arms and 4 legs.
I think it's just bad copy paste. They copied over the old base forms from the PRD, but didn't notice they had replaced chunks of them with new sub-type feature. I'm sure the intention is to ignore the old stat blocks, though technically you would have both sets of evolutions.
My suspicious is that the subtypes used to be optional, and things got messed up when they decided to make them required.
Another way to look at it is that it mentions arms and legs in addition to the slam. So if you still retained claws, that means you would also have 4 arms and 4 legs.
The base form doesn't list any limbs evolution.
The only evolution listed is "claws".So you wouldn't have 4 arms and 4 legs.
In fact, this appears as an argument against your interpretation.
Just found the answer from Mark Seifter.
The base form attacks are replaced by the subtype's evolutions.
https://paizo.com/products/btpy9c25/discuss&page=49?Pathfinder-Roleplay ing-Game-Pathfinder-Unchained#2407
Your link doesn't seem to work. But I feel we were always sure of the actual intention and developer posts aren't "official" anyway. But really, Unchained was just a toolbox of suggested house rules. So, your GM should feel extra free to rule however he wants whenever using things from that source.