How could Hinduism preach an impersonal god?
It does and it does not. An amorphous answer to your question above.
Probably the world’s most famous scholar of world religions and one who always put each respective religion in the most positive light, Huston Smith writes of Hinduism’s God in his book The World’s Religions:
“God so conceived [eternal contemporary] is called
Saguna Brahman, or God-with-attributes as distinct from the philosophers’ more abstract
Nirguna Brahman, or God-without-attributes.
Nirguna Brahman is the ocean without a ripple;
Saguna Brahman the same ocean alive with swells and waves. In the language of theology, the distinction is between personal and transpersonal conceptions of God. Hinduism has included superb champions of each view; but the conclusion that does most justice to Hinduism as a whole …is that both are equally correct. At first glance this may look like a glaring violation of the law of the excluded middle. God may be either personal or not, we are likely to insist, but not both. But is this so? Intrinsically,
God may be capable of being two contradictory things – we may say not because logic itself may melt in the full blaze of divine incandescence. But concepts of God contain so much alloy to begin with that
two contradictory ones may be true, each from a different angle. On the whole India has been quite content to encourage the devotee to conceive of
Brahman as either personal or transpersonal, depending on which carries the most exalted meaning for the mind in question.” (pg. 61-62, emphasis mine)
Now that’s one positive analysis. For me, God is not capable of being two contradictory things but Hinduism is very good at melting multifarious concepts into one amorphous whole IMHO. If you prefer “transpersonal” to “impersonal”, O.K. but others would simply say “impersonal”.