SYCL isn't based on OpenCL.

SYCL (SYCL-2020 spec) supports multiple backends, including Nvidia's CUDA, AMD's HIP, OpenCL, Intel's Level-zero, and also running on the host CPU. This can either be done with Intel's DPC++ w/ Codeplay's plugins, or using AdaptiveCpp (aka. hipSYCL, aka openSYCL). OpenCL is just another backend.

It is also a very long way from OpenCL C++. The code is a single C++ file, and you don't need to write any special kernel language. The vast majority of SYCL is just C++, so -if you avoid a couple of features- you can use SYCL in library-only form without even any special compiler! This is possible for instance with AdaptiveCpp.

Now, but that isn't how it started after the OpenCL 3.0 reboot, where 3.0 == 1.0.

Also some of that work, we have to thank Codeplay for, before their acquistion from Intel.

It's pretty much based on the same underlying featureset. Which is why trying to target Vulkan Compute from it is messy enough, whereas OpenCL is a natural target.