Sadly front end, like "High Level" is a very relative term. For example, in compiler design, the bit that parses code is called the "front end" since the "back end" is what emits machine code. I think that's what they mean here, the "front end" that understands D3D8 code has been added, presumably there is also a "back end" that converts the parsed/analyzed D3D8 code into valid opcodes for consumption by GPU/CPUs.

In the other direction, a UI/UX is sometimes called a "back end" when it is part of a more complex embedded project where physical controls are the "front end".