The whole point of that $1M is to send send the message "stay out". It's like a rattle snake shaking it's tail. Basically anyone doing this has to treat that money as throw away money since Apple may keep that money for whatever reason, including criticizing them on current/future TOS policy changes.
Alternative take: the whole point of €1m is to send a signal “I’m serious about doing this”.
By giving that letter to Apple, you're just telling Apple "I could give you $1M but I'm not going to, instead take this letter that says that I could give you $1M if you wanted to."
No actual money ever changes hands.
You can either have the letter of credit or be a known quantity. Presumably it you launch your marketplace and it goes well, after 2 years you don't need the letter of credit.