It says EBS can only be attached to one volume at a time but the aws documentation says theres ebs multi attach?

You can attach an EBS volume to multiple EC2 instances (called Multi-Attach), but that is a very specific use case with need-to-know constraints, such as:

• Limited to Provision IOPS SSD type only (io1 and io2).

• Limited to us-east-1, us-west-2, eu-west-1, and ap-northeast-2 if using io1 volumes.

• Limited to those supported regions of Block Express volumes for both io1 and io2.

• More limitations as described in Attach a volume to multiple instances with Amazon EBS Multi-Attach

So generally speaking, an EBS volume is single-attach, with exception of multi-attach under very specific conditions.