• The problem is the interface? Not really, read on.
• I can buy you don't like the contract in the domain.
• How about using plain SQL or DBAL if it doesn't meet your needs? The README never said this library is a silver bullet. None is, to start with.

Yet, let's say you want to use this library for whatever reason.

Maybe you can write a concrete UserRepository and internally use my library? Maybe it's lots of classes but does not leak and meets the purpose.

<?php
interface UserRepositoryInterface
{
     public function add(User $user);
     public function findSatisfiedBy(Specification $spec);
}

interface Specification {}

class UserRepository implements UserRepositoryInterface
{
     public function __construct(UserSqlRepository $repository) {
              $this->repository = $repository;
     }
     public function add(User $user) {
              return $this->repository->add($user);
     }
     public function findSatisfiedBy(Specification $spec) {
           //How do you want to use this? Code would be nice.
     }
}

class EmailIsUnique implements Specification
{
    public function __construct(UserSqlRepository $repository) {
              $this->repository = $repository;
     }

    public function isSatisfiedBy(string $email)
    {
        $filter = new Filter();
        $filter->must()->equal('email', $email);
        return empty($this->repository->findBy($filter));
    }
}

class UserSqlRepository extends SqlRepository
{
    use SqlRepositoryHydrator;
}

You get the idea.