Yes, the Buick emblem on the trunk is correct. For '70 they were only used on the ultra-low trim level Buicks like this one that have the plain back bumper that do not have the big "Buick" plate bolted on the bumper. I guess Buick did this a cost saving move? The small emblem was cheaper than the big Buick plate that would be bolted to the bumper. And they didn't want to sell a car that didn't have their name or model mentioned on the back.
I like it without the emblem. Nice and clean. However,it might be cool to retrofit some type of correct-appearing emblem or decal there,ex. Stage1,Stage2,GSX,etc.