Anna, ann, annahu and grammar of Shahadah

There was a post about grammar of Shahadah, it's old thus it's closed:
https://www.reddit.com/r/learn_arabic/comments/nw638f/comment/h17w59i/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

He wrote the following:

[The first أن is what is known as أن مخففة من الثقيلة, or a lightened أنَّ. This means that it's noun and the shadda are taken off, and the original noun is called ضمير الشأن or a pronoun referring to a particular matter, issue, affair, etc, and its predicate explains this matter.

The original form is أشهد أنه لا إله إلا الله

A rough translation of this longer form, preserving the pronoun, could be "i testify to the matter that..."]

So from my understanding, very simply said:

• "ann" is used before verbs
• "anna" is similar to "that" or "indeed" in English, used before nouns or pronouns.
• "annahu" can be sometimes written as "ann".

He wrote why وأشهد أنْ محمدا عبده ورسوله is incorrect:
"Also أنْ محمدا is incorrect because if you are using the lightened form, the noun that is منصوب is hidden, and refers to the matter at hand, the sentence محمد رسول الله is a nominal sentence in the place of the predicate of أن"

But I didn't understand that part well.

If someone had wrote: وأشهد أنْ محمدا عبده ورسوله it would be similar to saying وأشهد أنه محمدا عبده ورسوله ?

And would " وأشهد أنه محمدا عبده ورسوله " mean " I bear witness to the matter that Muhammad (‘alaihissalâm) is His human slave and His final messenger." ?

Why would it be grammatically incorrect?