According to Theoi, Plutarch was taking legends and myths and trying to approach them historically.
Romulus (along with his twin brother Remus) was one of the legendary founders of Rome, and his mythological origin was that he was born to a Vestal virgin who was raped by the war-god Mars:
As Silvia one day went into the sacred grove, to draw water for the service of the goddess, a wolf met her, and she fled into a cave for safety; there, while a total eclipse obscured the sun, Mars himself overpowered her, and then consoled her with the promise that she should be the mother of heroic children (Serv. ad Virg. Aen i. 274; Dionys. ii. 56; Plut. Rom. 27).
Theseus has a historical background as a prince, but other myths claim he's the son of Poseidon:
According to the commonly received traditions Theseus was the son of Aegeus, king of Athens, and Aethra, the daughter of Pittheus, king of Troezen. Other legends, however, maintained their ground, which represented him as the son of Poseidon by Aethra. (Plut. Thes. 6 ; Diod. iv. 59; Paus. i. 17. § 3; comp. AETHRA.)
So I think the quote you're referencing means that the author thinks Plutarch looked at the myths around these two men, realized they were born out of wedlock or their parentage was questioned, and suggested that myths grew to explain how such legendary people could have humble or unknown beginnings.
