The Muslim quoted and asked:

Psalm I

1 Blessed is the man who does not walk in the counsel of the wicked orbstand in the way of sinners or sit in the seat of mockers.
2 But his delight is in the law of the LORD, and on his law he meditates day and night.
3 He is like a tree planted by streams of water, which yields its fruit in season and whose leaf does not wither. Whatever he does prospers.
4 Not so the wicked! They are like chaff that the wind blows away.
5 Therefore the wicked will not stand in the judgment, nor sinners in the assembly of righteous.
6 For the LORD watches over the way of the righteous, but the way of the wicked will perish.

Could you please explain me why it is not about Muhammad (pbuh)?

Why it is not about Muhammad? The question should be, Why it is about Muhammad?

Nevertheless … it is not about Muhammad for this reason:

Psalm 1 does not speak about any specific person, it is not a prophecy, it is a general exhortation to all people to be diligent in studying God’s law and to walk in uprightness and keep away from the wicked.

To say it specifically speaks about Muhammad is a misinterpretation of a text which speaks about general principles as if it were about one person.

This should settle the case. But if you want to be very specific, then let me give you a more literal translation of verse two:

But his delight is in the TORAH of YAHWEH, and on his TORAH he meditates day and night.

Muhammad did not meditate on the Torah, and did not delight in it. He didn’t know it because he couldn’t read it and was not taught it. Furthermore, the word translated “LORD” is the personal name of God, YAHWEH which was never used by Muhammad either. He did not know God as Yahweh.

Come to Christ O Muslims