No.
Artists do not always own all of the rights to their music. Just because an artists says you can, doesn't mean that they actually have the authority to give completely permission to let you actually use their music.
Music is licensed on the mechanical and the composition side, and their can be multiple and different parties for each of these. You need approval from all of them.
Most importantly: Tweets are not legally binding contracts. Even if an artist owns 100% of the rights to their music, they are not legally liable to uphold statements from social media. You need a formally signed contracts with terms of use which include the length of the agreement and processes for revoking such permissions.