Learn Spanish (Latin America): Él sigue susurrando, mejor dicho, es chismoso. - He keeps whispering, or rather, he is gossipy.

It acts as a reformulation marker, meaning it lets the speaker correct or clarify what they just said. Here, the speaker starts by describing an action (susurrando), but then uses mejor dicho (literally better said) to jump to a more blunt or accurate conclusion: he is not just whispering, he is a gossip (chismoso). In English, we usually say 'or rather' or 'I mean.'

Since this lesson also covers más bien, could we use it instead of mejor dicho here?

Yes, but the nuance shifts slightly. Mejor dicho is used to correct yourself or find a more accurate way to phrase your own thought. Más bien is often used to contrast two things, leaning closer to 'instead' or 'rather.' In this sentence, mejor dicho feels like a spontaneous self-correction, whereas más bien would sound like you are contrasting the act of whispering with the fact that he is a gossip.

Why is it sigue susurrando instead of mantiene susurrando for 'keeps whispering'?

To express that an action keeps happening or continues, Spanish uses the verb seguir followed by the gerund (the -ando or -iendo form). So, sigue susurrando is the standard way to say 'keeps whispering.' While you can use continuar (continúa susurrando), seguir is much more natural and common in everyday Latin American speech.