No, you have misinterpreted kamui.
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As you can see from the above scan, one the right kamui eye is activated anything that is
about to come into contact with the user is phased through, Obito had to make a conscious effort to not become intangible.
When DMS is used, this ability is activated but the difference is the kamui user can choose to make any attack they want tangible as shown with kakashi while their body is safely intangible
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Kakashi not only pahsed through kaguya but the left eye ability was applied to his raikiri, what he came into contact with was warped. This means that no defense can stop the technique.
If obito had DMS minato would not have landed a hit and would have died as soon as the battle began. Indeed everything comes down to reaction, but I am comparing just the two jutsu not their users.