That thread is full of FUD (Fear, Uncertainty and Doubt) over an incident from 2015 when there was an integration (now removed, in a desktop environment that is no longer used) that would allow you to search amazon store from your desktop.
Ubuntu is just as trustworthy as any other major linux distro.
Because I did not follow through the instructions procedure upon installing I'm kind of reluctant to use TWRP now it is installed (only changed 'Keep System Read Only' into allow changes) , frankly I'm not sure whether it'll do more harm than good going through all options available. Too scared of the unknown I guess.....
How to check if I still need to change that into ext4 format?