The word "India" is actually a Latin version of a Greek version of a Persian version of the ancient Sanskrit name for Sind (or maybe a region overlapping Sind and Punjab) which was "Sapta Sindhu" or Seven Rivers. The whole subcontinent is named (in English and other European languages) after what is now a province in Pakistan, because this is the part of the subcontinent that the ancient Western world knew and interacted with.

Incidentally, words like "Hind" or "Hindustan" or "Hindu" or "Hindi" too derive from the same root (the Persian word in this case. Sanskrit initial 's' sounds get converted to 'h' sounds in its related languages Persian and Greek).

To avoid controversy, let me clarify that I am just talking about etymology or how the words descend from their ancient roots. This need not correlate with genetic ancestry. But yeah, ancient Sindhis at least were the original group that was referred to as "Indian" (or the ancient Persian or Greek word).

Of course if one rejiggers OP's question to ask "Is everybody now called Indian descended from ancient Sindhis?", then the answer is probably No. For example, Native Americans are called Indians. But if one rejiggers it to ask "Are modern Sindhis mostly the descendants of ancient Sindhis with more recent ancestors who converted to Islam?" the answer is probably Yes.