Comparison of Nirvana, Tao and Brahman/Atma
The Hindus had developed Samadhi (Concentration / Mastery over the Mind / Higher Mind) up to the the 8th Jhana, and also had a strong moral basis, but Buddhism went one step further and introduced Wisdom and Insight to the practice. The fact that many contemporary practitioners are strong in virtue and concentration would be the reason they manage to develop insight very quickly after meeting the Buddha.

So the practices of virtue and developing a higher mind does pay off in progressing on the Buddhist path also and will help when you switch to the developing Insight and Wisdom. But to go all the way to Nirvana you have to practice Wisdom and Insight which is only found in Buddhism, in the right and complete form. This was the discovery of the Buddha.

Also Buddhism has no notion of Soul / Atman or any unchanging core as taught in Hinduism.