Even when the subject is already identifiable, explicitly mentioning it may help lodge the statement more firmly in the hearer's mind. The text calls this additional clarification and reinforcement (زيادة الإيضاح والتقرير). Clarification increases explicitness; reinforcement stabilizes the intended judgment.
أو زيادة الإيضاح والتقرير
Or to make the meaning clearer and more firmly established.
أُولَٰئِكَ عَلَىٰ هُدًى مِنْ رَبِّهِمْ وَأُولَٰئِكَ هُمُ الْمُفْلِحُونَ
Those are upon guidance from their Lord, and those are the successful.
The second occurrence of «أُولَٰئِكَ» gives prominence to the second judgment. A pronoun could link the clauses, but repeating the demonstrative reinforces the judgment of success.