I mostly agree with you on "website/app". As for the platform, it really depends on the nature and size of the platform. If it becomes an essential platform or if it is a monopoly, then you could argue it has utility features and hence everyone has a right to it. For example, you wouldn't say a power or water company doesn't have to serve you because you can go start your own power or water company.
You could argue that twitter is an essential platform since its the primary form of communication for the president. It is a platform for you to communicated with your elected official. Hence twitter as a platform is a "utility" and cannot block US citizens from accessing it and exercising their 1st amendment rights. This actually could be an interesting Supreme Court case if anyone who has been banned wants to ligitate. It could actually be precedent-setting.
You could argue that facebook, youtube, etc are monopolies that nearly serve the purpose of a utility and hence you have a right to it.
Also, you could argue that these american companies have a duty to follow the "spirit of free speech" in our country even if they aren't legally bound to it.
There are arguments to be made on both sides. Unfortunately, it isn't being taken through the legal system.