Please note there are two buddhaas. The one who preached Shoonya Vaada and the one who preached the correct version for the sajjanaas. After Dwapara yuga, devataas had gone back to their respective lokaas. Sri Vedavyasa Devaru was invisible.
Then Tripuraas who were killed by Rudradevaru were born again in the disguise of Vaidikaas and as such they had access to Shastras.
One among these Tripuraas was again born as the son of Shuddodhana. This Shuddodhana got a baby and he was performing vedic jata karma to the baby. At the same time, Srihari made that Baby to disappear and he himself came to that place.
The new child (Srihari) who replaced Siddartha did the upadesha which were against Vedaas. To further convince them about the same, he told devataas to throw weapons on the child. He swallowed all the weapons including Rudradevara Trishula and sat on Vishnu Chakra. Then he started preaching “Shunya vaada” (The same shunyavaada is represented by Advaitees in a different manner). As Srihari’s budha roopa sat on the Vishnu Chakra, they thought that this child superior to Vishnu and followed his preachings.
Please note : The preachings by Srihari’s Buddha roopa is only for asura jana mohanaartha.
Some of the teachings by Buddhavatara roopi paramathma with dual meanings, one for the asuraas and one for the sajjanaas
Veda: apramanam –
meaning vedaas are not having any pramanam (for the asuraas)
meaning “a”karachavachya paramathma’s is pramana as per veda (for sajjanaas)
The world is shoonya (for asuraas)
Sham – sukha; u + u = of high relevance + svaabhivika, shoo – That god
Neeyate – niyamana ( Srihari does the niyamana of the entire world and he has sukha, which is svaabhivika) (This is for sajjanaas).
abhaava – The entire world is brought up by akaara vaachya paramathma
asat – The entire world is destroyed by akaara vachya paramathma
kShaNika – Srihari’s word are prameya siddha
The words Shoonya, abhaava, asat, kshaNika were misrepresented to asuraas and represented to sajjanaas in a different way.