Interesting question. It depends. I linked Ev Fedorenko's Interesting Brain Project at MIT up above, they're doing a deep dive into questions like those.

Broadly speaking, if you're born with these anatomical anomalies, you'll be more or less normal. The article mentions the person in question had an IQ of 70, so that's lower than normal, but not intellectually impaired.

But acquired Brain damage almost always leads to impediments. Strokes and repeated concussions, physical injury, etc.

The brain is "plastic" when you're young, we like to say. That is, it's pliable and can mold into whatever shape it needs to in order to adapt to your environment. That plasticity disappears once you get older. It's how kids can learn language effortlessly - when you're born, you have the most neurons and synapses you'll ever have in your life. You'll keep the same neurons (unless you have a degenerative disorder or kill them with drugs), make new synapses as you learn, but broadly speaking as you grow up you prune synapses that aren't helpful.

This is also why kids can undergo massive resection surgeries (or in the olden days, severing of the corpus callosum) and grow up more or less normal.

intelligence is a very hard thing to quantify. People who are good at math might be terrible at writing plays, both of which are intelligent processes.