I don't believe it's actually Magic, but more that it's Ancient Technology mixed with his Devil Fruit that Imu is using.
Possible. The true difference in magic and science is apparatus. Magic typically uses individuals themselves as apparatus for changing energy (ie, a Mage spewing flame), while Science involves artificial apparatus to invoke changes in energy states.
That's all science and magic are; manipulating energy states to get desired results.
Science is more order based, based in numbers and stringent calculation.
Magic is more chaos based, based in symbols and feelings.
Any advanced science will be viewed as magic by ignorant people, but base magic comes without using a 3rd party.
IE, Franky, a Warrior of Science, needs tech to spew fire.
Ace can simply point his hand out and cast flames on people.
Magic is more holistic, science is more technical.
IMO what really made the Ancient Kingdom dangerous is they began to bridge the gap. Just technology and just magic isn't too dangerous.
Science that allows for Magic, like the Devil Fruits, are truly dangerous.
A man that can spew flames from his mouth through magic can be equally as dangerous as a child with a lighter.
A man who can build Fruits that give children the ability to light things on fire? Science mixed with magic? That's dangerous.
I detest Imu, but I understand their perspective. Giving people that much power can lead to things going very bad very quickly?
V. Nujsuro touched on it best, IMO:
Vegapunk's science is dangerous BECAUSE it pushes science into the realm of magic.
I mean, look at the Seraphim. Why are they so strong?
They are the Magic and Science in lockstep, genetically engineered super-humans who can fly, have laser tech, all on top of Lunarian genetics reinforced with cyborg technology + Paramecia Devil Fruits.
Imu fears the Void/Ancient Kingdom/Vegapunk because their combination of Magic + Science threatens Imu's Magic. Look at Mary Geoise: it's WAY behind Egghead. Imu clearly dislikes science, he's the richest man in the world, and he's done nothing to really update Mary Geoise with advanced tech.
Imu fears Magic mixing with Science because it can bust through Imu's powerful Magic.
Hence why Imu fears Nika/Joy Boy: they are the Magic of a God trapped in the Science of a Devil Fruit, a ticking time bomb with a will of its own that literally evades the WG through legitimate plot armor.
We can also see the Straw Hats exemplify this mindset: Franky loves science, but is also very nature focused. Robin loves the science of archeology, but her Fruit is very "Magical" and she is referred to as a Demon, literally becoming one.
Nami is a "Witch" who stole a talking Thunder Cloud from a Yonkou, and fused it with advanced weather technology provided by Usopp + Weatheria. Sanji is a "Warrior of Science" who loves cooking/love, but was created to be a heartless soldier through genetic experiments.
The Straw Hats themselves are a threat to Imu's Magic, because they combine Science and Magic effectively.