Earlier quoted context omitted.
Sorry, but this protocol should be improved: "in" is ambiguous. All the startup has done is repeat verbatim what the investor said. In this situation the startup has signaled receipt of the investor's offer and has not signaled intent to ratify or otherwise accept.
"You're in for [number]" is meant to be interpreted as acceptance.
>1. The investor says “I’m in for [offer].”
>2. The startup says “Ok, you’re in for [offer].”
You'll notice that the startup has only repeated back what the investor said, correcting for the inverted subject-verb and adding a customary signal opening. The startup has confirmed receipt of the investor's signal.
>3. The startup sends the investor an email or text message saying “This is to confirm you’re in for [offer].”
The startup confirms receipt on a secondary channel.
>4. The investor replies yes.
Waiting on communication from startup.