FI is much easier to define than RE. FI is a mostly a pecuniary matter... dollars in, dollars, out, investments to support the transaction. For some of us here, there's a long runway between FI and RE, possibly even decades.
Retirement was easier to define in the day of.... defined (not coincidentally!) benefit pensions. Upon receiving such a pension, a person become retired, even if immediately going for a second full-time job. Right? The "retirement" was less about cessation of remunerated labor, than of achieving some badge of seniority and recognition of labor. Even today, we speak of military officers who did their full-20, and then retired... as "retired".
Example: Smith reached O-5 in the Army, retiring after 23 years of service, at age 45. Smith realized that he won't make O-6, and therefore declined his final relocation offer, choosing to retire instead. Knowing that he's due for a decent pension, Smith never did save much money. He's not FI. A few weeks after leaving the Army, Smith went to work full-time for Lockheed Martin. Though his cubicle-mates regard him as just another employee, Smith can legitimately call himself a "retired military officer". And we thank him for his service.
On the other hand, Jones started at Lockheed Martin right after getting his electrical engineering degree at Georgia Tech. He was at Lockheed for 28 years, until getting downsized, at age 50. Jones saved his nickels, investing them in the S&P 500, so he's comfortably FI. But feeling distraught at losing his job, Jones returned to GA Tech as a part-time adjunct professor. He's been there for 5 years now. Is Jones retired?
In this vignette, Jones is nearly a decade older than Smith, has considerably more money, has more years of full-time employment, and is currently working fewer hours a week than Smith. Even so, I'd say that Smith is retired, but Jones isn't.