That's not the relevant bit, though. I asked about in the rules. You gave me Sage Advice.
Which is literally the guys that designed and wrote the rules, telling you what the rules say.
If a rule, effect, item or ability expressly states the rule, ability, effect or item is magical (using the words magic or magical or magically) then the rule. effect or ability or item is magical. If it doesn't have those words (and its not a spell, doesnt replicate a spell, or isnt a magical item), its not magical for game rules purposes.
The writers are consistent in this when designing abilities. If your ability, effect or feature is designed to be magical, it will expressly state it in the description of the ability, effect or feature.
Look at abilities like:
Wild shape: Starting at 2nd Level, you can use your action to magically assume the shape of a beast that you have seen before.
Eldritch Invocations: In your study of occult lore, you have unearthed Eldritch Invocations, fragments of forbidden knowledge that imbue you with an abiding magical ability.
None of those work in an AMF and they can both be detected with Detect Magic.
Compare to abilities like:
Hurl through Hell: Starting at 14th level, when you hit a creature with an Attack, you can use this feature to instantly transport the target through the lower planes. The creature disappears and hurtles through a Nightmare landscape.
At the end of your next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic damage as it reels from its horrific experience.
Hurl through Hell is NOT 'magical'. It cannot be detected or dispelled as such, and works fine in an Anti Magic Field (barring the prohibition on extradimensional travel in an AMF which blocks it for other reasons).
Ditto features like:
Dragon Wings: At 14th level, you gain the ability to sprout a pair of Dragon Wings from your back, gaining a flying speed equal to your current speed. You can create these wings as a Bonus Action on Your Turn. They last until you dismiss them as a Bonus Action on Your Turn.
You can’t manifest your wings while wearing armor unless the armor is made to accommodate them, and clothing not made to accommodate your wings might be destroyed when you manifest them.
Doesn't say its magical, so its not magical. A Dragon Sorcerer can instantly sprout full blown Dragon Wings in the middle of an AMF and simply fly away.
The difference is whether you should state it as Teh Trvth! or as what seems to you to be a really good way to work with it, given the ambiguity in the rules.
The rules are not ambiguous. Only things that are expressly described as magical, are magical.
Ghosts, Dragons, Gremlins, Golems and Demons are magical/ enchanted/ clearly supernatural creatures, but are not magical creatures for the purpose of the rules.
I'm simply stating the Rules as Written (and as intended to be read and used by the guys that wrote them).What you've laid out is a reasonably consistent way for a GM to call it, which is great. But I see nothing compelling about it that suggests that a GM would be unreasonable to decide differently, and nothing about calling it differently that breaks the system.
Its super important if you want to write homebrew stuff (abilities, archetypes, class features, feats etc). If the ability is magical, you need to state it in the actual ability description.
As an example from my own homebrew Battlemaster manoeuvres (converts of three ToB manouvers for 5E):
Sudden Leap: As a bonus action, you expend a superiority dice and leap up to 20’. This movement does not provoke attacks of opportunity. If you make a melee weapon attack immediately after this movement, it has advantage, and you may add the superiority dice to the attacks damage if it hits. You cannot use this maneuver if your speed is 0 or you are otherwise unable to move.
Holocaust Cloak: You cloak yourself in magical flame as a bonus action, expending a superiority die. Until the start of your next turn, any creature that strikes you with a melee attack takes fire damage equal to a roll of your superiority die. A creature can only take this fire damage once per turn.
Crusaders Strike: When you hit a hostile creature with a melee weapon attack, you can expend a superiority die and magically heal an allied creature. When you do so, choose a friendly creature who can see or hear you within 30'. That creature regains hit points equal to the superiority die roll.
One of those manouvers can be used in an AMF and detects as magical. The other two, cant.
Can you tell which two dont work (and are magical), and why?