Salut Maithilee,

The simple answer to your query "Why not Ils seuls sont venus ?" is that the lesson is about stress pronouns and "ils" is not a stress pronoun it is a subject pronoun.

The appropriate stress pronoun for "ils" is "eux"     "They are the only ones who came" or "they alone came"

Hope this helps.

Bonne continuation

Jim

Jim's right, eux is the stress pronoun of the 3rd person plural (ils).

Ils sont venus seuls. They came alone. No stress pronoun needed since it's a simple and neutral statement.
Eux seuls sont venus. -- They alone came. Here the stress is on who really came, hence the stress pronoun. The clue is the placement of the adjective seuls.