Author here. All good points from this point and its replies, which to me says best to me that not all products and experiences are equal, so proceed based on your comfort level.
Note that the app is capable of monitoring both battery charge percentage and temperature. If one was inclined a USB to serial adapter with some simple circuitry could be used to monitor and control the state of charge. Good idea for a follow-up project and article!
There are apps that can limit the charge %, or using something like Tasker to cycle a smart plug.
Android 16 also has native limiting under Settings > Battery > Battery Health > Charging Optimization > Limit to 80%
I've had a phone with a swollen battery that I swear was not swollen when I chucked it in the drawer a year or two earlier. Apparently it's the battery itself breaking down that causes this, so I don't think any electrical protection scheme would necessarily help you.
I guess my advice is "don't use a really old and worn out battery for this". Otherwise it's probably fine? People walk around all day with one in their pocket and sometimes their ear after all.