First of all, as you have been told many, many times, the constitution applies to the government.
Private business is not obligated to respect free speech rights.
Second, what does apply to business is contract law.
This is a case of simple contract law.
When mtg signed up on Twitter she signed a contract with the company that she would follow all their TOS rules and if she didn't she gave Twitter permission to suspend her account or even close it if she violated that contract.
mtg violated that contract. So Twitter exercised their right to enforce their contract with her and suspend her account.
She gave them permission to do so when she signed the contract.
If you want free speech laws to apply then you need to do your free speech with the government. The government can't prohibit your free speech rights.
A private company most certainly can especially when the client signs a contract giving the company the right to do so.