Hello, let it go out. is an idiom and direct order. though in simple present tense singular subjects takes s, es, or ies, we can use do with singular subjects. like: stand up, open the door. wash your face. open your book, or let it go. let him swim.
For more, you can check it
https://carvetheraw.com/simple-present-tense/
Simple Present Tense
Simple Present tense is a verbal tense that is used to express present action (an action that happens in the present), habitual activities (an action that we do every day owing to our habits), daily activities (everyday work), universal facts (things which can not be changed by human beings) or possession and state (things or activities which are general truth). The simple present tense is formed by adding s, es, or ies to the end of base verbs when singular nouns and pronouns appear ( he, she, it singular nouns). If the Subject is plural(They, we, and you) we do not add -s, -es, -ies to base verbs.
For example, a student says,
“I go to school”. It is a daily activity of a student to go to school, so the simple present tense expresses such actions.
Another example is, “I work in a factory”. It tells about the usual actions of a person that he works in a factory on regular basis.
I feel tired. It looks beautiful. He enjoys the party. (Happens to write now or general truth)
Note: we should not say this:
I am working in a factory. It tells about continuous action, and for that, we use the present continuous tense.
Simple Present Tense Usage
Simple Present tense is used for habitual action, universal truth, daily activity, and general truth. Let’s study them one by one with examples and usages.
A ) Habitual action: any action which we gain by habit.