...because what it is not does not exist, and I cannot have a concept for something that doesn't exist.

This just begs the question of how you define "exists" (concrete versus abstract).

This brings us back to Quanta versus Qualia.

Does NOUMENON "exist" (since it's not "empirically verifiable" but remains logically necessary)?

...falsehood cannot be "not truth", but it is what is subjectively true, although not objectively true. Unlike what we usually call truth, which is objectively true, out of appearances.

You've highlighted another important misconception.

The "objectivity" trap (humans are fundamentally subjective).

Think of it this way.

There are a very few things that can be considered "provably true" (apodictic truth).

There are a very few things that can be considered "provably false" (logically incoherent).

EVERYTHING ELSE (99.999%) IS QUALIA (OPINION, GNOSIS).

I also see the danger of considering any knowledge as "irrefutable facts"...

That's why it's important to clearly define the very exclusive and rare scope of apodictic truth.

...because, one is always human and there is a considerable margin of error, so that what we consider "facts" can also end up being opinions,

This should never happen. Facts are always distinguishable from opinions.

...that's why I am more on the side of falsificationism and I tend more than anything to consider justified knowledge as "not refuted", rather than as "absolute truth".

I consider "justified knowledge" as "actionable hypothesis" based on its EFFICACY.

I see no utility in conflating "actionable hypothesis" with apodictic truth.