You say this physical activity has meaning for "you", but "you" are also a "particular cascade of activity", are you not? Even a rock on the ground is activity, on the atomic level and on the cosmic level. So how does one cascade muster the ability, not to mention the presumptiveness, to make a value judgment about another cascade and think that it can be meaningful or true?
As I said, it's a question of information processing. Meaning is the label we give to a form of associative information processing - and yes, the consciously aware 'you' itself consists of a particular kind of information processing, as are the value judgements you make, and whether you consider something to be true or not.
If you're asking how it can be that there is 'something it is like' to be all this information processing going on in brain and body, that's the 'hard problem' of consciousness, for which there is no current solution.
But some well-informed opinion compares it to the problem of life itself in the time of Vitalism (19th century). The study of the property of 'being alive' was expected to find some substance or spark, an explanatory
elan vital. But, on investigating the physical and chemical properties and mechanisms of living things, it slowly became clear that there was no
elan vital - the structures and functions of living things were emergent properties of complex organic chemistry.
IOW, it may be that, for systems with a certain structure and organisation, when processing information in particular ways, there will be 'something it is like' to be that system.
I think that's all I have to say about this. There is no scientific basis for this extreme reductionism, as I said earlier it's a belief, and as such I don't know how to argue against it any further than what I've said.
Perhaps some people have it as a belief - personally, I think it's a reasonable scientific hypothesis, given that all the empirical scientific evidence gathered in recent years increasingly points that way, and we have a good physical understanding of the stuff we're made of and what can influence it. If we find evidence that there must necessarily be some special or mysterious 'spark of consciousness' that cannot be accounted for by the physical forces, atoms, & molecules we're made of, then we'll have something even more interesting to investigate.
The argument from incredulity is a form of fallacy...