Ehh, its a bit more than that.
Its a particle in that we know they are quantized into single photons. As in, it is impossible to observe half of a photon, or any non-integer number of photons, and one photon can only be observed in one place. This makes it like a particle.
But its a wave in the way it behaves - it can interfere (not just with other photons, with itself), and its movement can only be described through wave functions that can even take seperate paths at the same time, according to how waves propogate.
And, there are ways in which they act like particles no matter how they are observed, and same for wavelike behavior
Worth noting: "observation" is just physical measurement. You have to keep in mind that observing something fundamentally requires interacting with it - in order to look at an apple, photons must bounce off of it, which is a physical interaction. On the quantum scale, these interactions cannot be ignored.
Also also: this isn't just photons, everything is like this. It may not align with how we observe things on a macroscopic scale, but this is fundamentally how the universe works.
I really hope valve can get VAC into a good space so it can be an example of doing anti-cheat well without having to be a rootkit. Looks like we're heading towards that point. Hopefully this change will last