The Foreign Service Institute training is also full-time, 8+ hours a day, working with dedicated tutors in small groups. Basically, it's your job to learn the language.
This isn't taking a semester of high school language instruction or playing with Duolingo for six months. It's basically doing nothing else during working hours and maybe homework for over half a year in what would have probably been close to a $100K crash course.
They mean 24 weeks in a residential, 24 hour a day program, where you sign an oath never to speak in anything but the target language, and which is largely attended by people in the armed forces whose career will be affected by their performance. The level is probably about B2/C1, and they expect these people to immediately go to work in the language.
English shares a lot of vocabulary with French, so that's a lot less for English speakers to learn.
These are the estimates for how long it takes to reach the FSI "Speaking 3: General Proficiency in Speaking (S3)" and "Reading 3: General Professional Proficiency in Reading (R3)", which I understand to mean roughly B2 in the CEFR scale.
