Not to belittle the writer's efforts, but they went to a depth of....20metres?!
That's it?
With those big giant fins, combined with the weights, they should easily be able to go 1m/sec without it being any kind of hurry. So, that'd be 20sec each direction. Doesn't seem like it'd require "numerous classes" to "learn to breathe again".

Any half-decent 13yr old age group swimmer can easily go 25m underwater. Decent ones can easily go 50m. Offer some dumbass 15yr old a cookie and they could probably go 75m (speaking from personal experience).*

Make no mistake: I am in awe of freedivers and what they are able to accomplish - up to 5min underwater up to 200+m deep is literally incredible!
In this case, though, even if the writer did only go 20m deep....you'd have had my permission to embellish a little to provide a little more authenticity to your experience. ;)

*Anyone trying this, whatever you do, DO NOT (and this cannot be stressed enough) DO NOT! hyperventilate before attempting underwater breath-holding. Read up on "shallow water blackout".
There's a reason freedivers have a dive buddy and multiple checks with the diver even after they get to the surface. Blackout, in water, with very little residual blood-oxygen leads to brain damage and/or death very quickly.

This is a fantastic article about extreme diving. Not free diving, but diving using a rebreather. I read it years ago and I’m about to read it again.

https://www.outsideonline.com/outdoor-adventure/water-activities/raising-dead/