[Grammar] He will come/is coming to see us

Hi! could you please help me

He will come/ is coming to see us this evening

What tense does refer to arranging and being sure that a person actually comes? I always think that "is coming" is a 100% sureness and "will come" just a guess. But yesterday i was surprised when an interpreter said to me that it's wrong and "will come" means an arranged meeting.
Please help me to get rid of this cognitive dissonance in my mind!

"Will come" and "is coming" are both expressing 100% surety unless qualified in some way. For example, "I think he is coming". "She will come if she has time".

'The future is uncertain ... we can never be 100% certain about the future.'── quoted from https://www.englishclub.com/grammar/verbs-m_future.htm

Expresses surety.

Nothing mentioned about guarantees, just someone being certain of a thing.

ChinaDan said:

Expresses surety.

Nothing mentioned about guarantees, just someone being certain of a thing.

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That is only the case if the person believes, wrongly, that the phrase expresses a 100% certainty - or he'd express it differently, such as "I'm 100% certain that he'll come tomorrow".
In most cases, if a speaker says, "He's coming tomorrow", he both knows that this is not certain, as well as knowing that it doesn't mean that it is certain.

Raymott said:

That is only the case if the person believes, wrongly, that the phrase expresses a 100% certainty - or he'd express it differently, such as "I'm 100% certain that he'll come tomorrow".
In most cases, if a speaker says, "He's coming tomorrow", he both knows that this is not certain, as well as knowing that it doesn't mean that it is certain.

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The speakers intent controls the choice of words; not the validity of their opinion.