I can understand their perspective, especially if they're non technical. Much of the time they see open source as some ragtag community project that has no real support so they believe in terms of risk, it can seem lower to build or buy.
Without those assurances, and without investigating the project myself, I’d probably turn it down too.
Support contracts can be very lucrative.
I work with a company that generally stays away from open source software, unless they have an in-house developer that can maintain it.
Most open source projects (unless it's backed by a huge company that also charges for support) are run by volunteers that have a day job. This means that support will be non-existent beyond what the developer(s) feel like and bug fixes/updates may or may not happen. This doesn't work that well when a company relies on this software for any critical task.