Is there an input length range for SHAKE128 hash function?
The quoted paragraph talks about the testing of SHAKE128. As usual, for the 128-bit security achievement, you need to think about the generic attacks on hash functions.

For preimage resistance, you need 128-bit digest size. However, due to the birthday attack, you need 256-bit digest size to achieve 128-bit collision resistance. That is $\sqrt{2^{256}} = 2^{256/2} = 2^{128}$.

For the input length; if the input space is small, it can easily brute-forced or a special rainbow table can be implemented to find the preimage of the hash values. For 128-bit preimage resistance, you need your input space to be around 128-bit.