Definition of Hindu as a believer of Shruti-Smriti has some problems to it I feel which I will try to explain using a very dumb example.
Counters on this one are really appreciated.
Let's suppose I say I am a firm believer of Vedas, Shruti, Smriti and scriptures but I also
श्रुतिस्मृत्यादिशास्त्रेषु प्रामाण्यबुद्धिमवलम्ब्य श्रुत्यादिप्रोक्ते धर्मे विश्वासं निष्ठां च यः करोति स एव वास्तव हिन्दुपदवाच्यः।
- स्वामी श्रीहरिहरानन्दसरस्वती 'करपात्रीजी'
A Hindu, according to Karpatri Ji, the Gurudev of Puri Shankaracharya Ji, is someone who accepts the authority of the Vedic scriptures (Shruti and Smriti) as valid knowledge (pramāṇa) and has firm faith and commitment in the Dharma taught by them. Since nāstikas do not accept Shruti and Smriti as valid authority, can they fall under this definition of Hindu as given in that statement? There are, however, many definitions of Hindu. What Puri Shankaracharya Ji said is in the context of Civilizational Unity. Period.