This is false. License [0] clearly states the conditions under which you can do it and they seem pretty reasonable. Nothing that a normal user, faced with an issue they want to fix, wouldn't accept. I imagine Amazon would have trouble accepting those and other terms, but that's the whole idea.
No, you are both correct. You are talking about the SSPL, the parent poster you are replying to is talking about the Elastic License which does in fact prevent you from modifying the code and running it in production:
With the dual-licensing you have a choice of Elastic license or SSPL, but they both have terms you might not like. And if you use the binaries you automatically use the free as in beer Elastic license.
[0] https://www.mongodb.com/licensing/server-side-public-license