It’s not redundant because the scalable doesn’t have to be elastic. Not being able to remove resources is not an intrinsic feature of scalability, it can, and even if it cannot or is not done, this is not necessarily a problem. Being scalable doesn’t care about that stuff.
The problem is the definition of the term scalability. It is about the ability to meet higher demands, it does not determine how, it does not impose general requirements. A need for scale may impose specific requirements.
In addition, people understand scalability as the ability of servers to meet more demand. The term is much broader than that and can be applied in other contexts. We may be talking about software scalability.
The shape of scalar is not defined. One of these forms may be elasticity, it may be that what is scalar has no sense to have elasticity.
On servers we can scale vertically or horizontally.
To ease and speed of making this change of scale, both to increase and decrease will determine whether the system is elastic. Something elastic tends to use only the necessary resources. The need and change of scale can even be managed by software. In general something is elastic if it can be done automatically.
The cloud is a mechanism and a form of commercialization aimed at offering elasticity. Or should it be, in practice it’s usually better, for suppliers, not give so much elasticity, getting better billing and more profit even because one can pay for something that does not use.
What many people don’t realize is that being elastic doesn’t mean spending less. It’s very common to spend more, because it has a higher price. Much less being elastic is the best solution. If it were better and cheaper everyone used so.