Why is var used all the time in Functional Programming section?

I have been using let and const instead of var as I’ve seen this done in FCC previous sections, but in the Functional Programming I don’t see any lets and consts so far, it’s just vars. I’m wondering if there’s a benefit to using var here, or is the section a bit outdated?

the curriculum has been developed in waves, some sections are older than others

it is moving to a project-based curriculum which will also make it more uniform