Secularism and Sarva-Dharma-Samabhava are entirely different concepts. Secularism is a historical product of Enlightenment in the Western civilisation. It is based on the principles of humanism (only human beings have value and agency in this existence), empiricism (only truth is what can be known through human sensory perception) and rationalism (only truth is what appeals to human reason). In practical terms, it means every person’s life should be conducted by his or her own will (liberty); the state should be constituted through participatory processes (democracy) and should function on rational premises (secularism); and each person possesses same rights as any other (equality).
Sarva-Dharma-Samabhava is a Hindu idea, which means we must respect all religious sects and all occupations-stages of life (Varnashrama Dharma) as different paths to the same God. It is opposed to humanism because it believes that human beings are bound to Dharma and his or her whole life should be dedicated to fulfil that Dharma. It is opposed to empiricism because it recognises Veda-Smriti-Purana-Tantra-Guru as valid evidence to ascertain truth. It is opposed to rationalism as it considers human reason sub-ordinate to Dharma determined by higher authorities like gods, manes and seers. In practical terms, Dharmic duties replace liberal democratic rights as the essence of social organisation and Dharmic hierarchy discards the egalitarianism.