Despite his former job as chief of the FBI's counterintelligence section, Strzok had the naive notion that texting on FBI phones could not be traced. Strzok must have slept through "Security 101." Or perhaps he was busy texting during that class. Girlfriend Page cannot be happy at being misled by his assurance that using office phones would be a secure way to conduct their affair(s).
It would have been unfortunate enough for Strzok and Page to have their adolescent-sounding texts merely exposed, revealing the reckless abandon of star-crossed lovers hiding (they thought) secrets from cuckolded spouses, office colleagues, and the rest of us. However, for the never-Trump plotters in the FBI, the official release of just a fraction (375) of almost 10,000 messages does incalculably more damage than that.