Is burn out correlated to the number of tasks someone takes on at once? If you can do five tasks in parallel for 6 hours a day, I wouldn't think that would cause burn out.

Burn out is probably just correlated with the amount you overwork yourself. And just like before AI, great engineers tend to have a higher threshold for what they consider overworked.

Thinking about it a little more, the qualities that made an engineer great before AI are still very similar today, but weighed slightly differently.

Sequential implies a sequence, ie many tasks. One single task can't be sequential or parallel. Or you are talking about a feature that is broken in multiple sequential tasks.