If the person has an OTO test kit, they'll survive. The shades of yellow-orange-brown give some indication.
If the person uses test strips, the colors can bleach out and falsely indicate no FC when in fact there may be plenty, in which case they might bleach a vinyl liner and at minimum waste a lot of bleach
If they use a DPD test kit, the pink indicator can also bleach out with the same consequences
I know the thread you mean - and he wasn't working blind. He did have a test kit and diluted the sample to estimate FC between doses. Most of the people who try to do it attempt it with pool store or test strip CYA results, which are almost certainly wrong and can lead to overchlorination - wasted money - or underchlorination - a SLAM that lasts too long and wastes money and bleach; love pats rather than knockout punches.
Chlorine losses to the sun are not a fixed parts per million. Overdosing just leads to faster UV losses. Check the graph from this thread
Pool Water Chemistry
https://www.troublefreepool.com/~richardfalk/pool/ChlorineLoss.gif
Overchlorinating leads to more waste. The SLAM level we advocate are a balance of killing power, UV losses, and experience. Without a test kit, once could easily just add 50 FC and dump another 20 in every hour until everything is dead. A whole lot of pool services do just that... the Nuclear Option. They don't care if they shorten the life of the liner and the seals inside the pump and filter; more money for them down the line.
At the end of the SLAM, there's no way to run an overnight loss test just using colormatching. It's just not accurate enough. I could find countless threads here where the water is crystal clear yet the pool is still losing FC overnight, and it ends up being an algae colony hidden within the light or the ladder or something. By measuring the loss accurately and not stopping too soon, the algae is eradicated for keeps. There are equally countless threads whining that the algae came back after a SLAM and further discussion shows it was stopped too soon.
It's your pool - you can do what you want with it. But do not come into the forum whining that your liner is discolored, or it's taking too long, or that you "shocked" the pool and the green came back, or that it's taking so much bleach, because you'll get hammered mercilessly for it. How do you know if you're adding enough bleach to get ahead of the algae growth or just enough to make the green monster laugh at your puny efforts? If you need to add 15 and you add ten, it's not even a speed bum,p to the algae. The bleach went in and died for nothing. You just don't know, not without a test kit. And I pointed out at the beginning that the wrong test kit can't measure FC at the levels needed.