Truth is "not subjective" in the sense that force of will can't change the answer to even very fraught questions like "what programming paradigm works best in situation X" or "which running back would do best on team X in superbowl Y".

But one's judgement on the truth of complex question depends on one's opinions on the meaning of various terms, reliability of various individuals, the dynamics of human psychology, etc. This overall situation results in people's opinions being hard to compare and "truth is subjective" is often shorthand for this, although it would be nicer to have a different word.