Flareon learning Smog so late is — believe it or not — probably a good, conscientious decision in some ways.
Fire-Type Pokémon randomly learning Smog is a design choice they’re clearly quite tied to, but you have to imagine it’s only ever been intended as a cute flavour pick. In Gen I Poison-Type was only supereffective on Grass- and Bug-Type, both of which Fire-Type is too. After Gen I it’s only Grass-Type until Fairy-Type’s introduction much later, but we’ll focus on 1-5 (because new Fire-Type Pokémon don’t really get the move).
The real trouble comes when the odd Fire-Type is stuck with Smog as a move it kind of has to use. IIRC Gen IV Slugma, which you can get from the Primo egg in HGSS and I once used, was stuck with Tackle / Rock Throw / Smog for an incredibly long time. Ember came at something like level 17 off the top of my head, the same level as Bugsy’s Scyther. It makes sense to make the movepool that way, because Ember is a lot stronger than the other attacks so you should put it later in the learnset, but it also makes for an awful play experience.
Late Smog is the same as those mostly flavourful status moves that are often put late in a Pokémon’s learnset, IMO. The likes of Gastro Acid, Gravity, Magnet Rise are often put in learnsets late if they’re put there, and they fulfil the same role of “this Pokémon should learn this by level up but our options are level 1 for flavour most players won’t really see, in the middle of the game where the Pokémon needs a real upgrade, or way later as an Easter egg for dedicated trainers of the Pokémon who clearly like them”. The last option is the best one, particularly since that’s when players start caring about battle facilities, a context where in theory these niche moves are most useful.
Smog has a small niche on Fire-Type Pokémon, that being a worse Toxic. Against most Pokémon you’d use a move like Lava Plume if you want to status spread, but Fire-Type Pokémon (and like, Water Veil) are immune to burn, so if you’re really dedicated to status spreading, carrying a move to spread poison is a valid backup. It’s not a backup anyone would actually use in a battle facility unless it’s Toxic, but it’s not totally crazy to consider. This puts it in line with the other niche late game status moves Pokémon learn sometimes.
Particularly on Flareon, who can evolve at any point and doesn’t learn any STAB in its pre-evolved form, using a level up slot during the actual main game portion on Smog of all things is a downgrade. If you evolve at the wrong level, you can be put out from it actually being a Fire-Type for an even longer time.