In Polytheistic religions, there is always room on the bench to add more deities. You can see this in old Roman and Greek Pantheon of Gods too. The Vedic God Mithra found a place in Roman Pantheon. Even Zoroastrianism which is believed to be Monotheistic found a place for Mithra.Becasuse most of Indians consider Buddhism as part of dharmic religion. For most budha is just another one of god/guru, one among literally thousands. Heck, I know many hindus who worship Jesus at home like other Hindu gods like below
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Even Ramakrishna Mission, does puja to Jesus
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As far as most are considered they are just another god in the long list gods. I know multiple family members who are staunch Hindus who pray to all Gods including Jesus and visit Darga
So for most when they say Hindutwa they just mean their way of life and less about religion
Mithraism was quite popular in Persian Empire and Roman Empire before Christianity came into the picture.