There are no silly questions Hazel, only silly answers.
In essence your method is perfectly ok.
However, NO particle board is a good substrate.
Depending on the type of chipboard it may require a particular type of preparation before you can even fix Ditra 25 to it, some are resin faced for example.
To split hairs, ANY material placed between the substrate, in this case chipboard, and the finished floor surface will decouple the floor, as long as it’s only fixed to the chipboard and not through to the joist.
Basically the role of the decoupler is to separate it from the main structure. (Your home)
That way the building can move and your floor, will in theory, stay in tact.
That is the primary role of a decoupler.
In the case of a cement board, it will still act as a decoupler, as it’s placed between substrate and tile.
What a cement board would have over a membrane is the fact that it’s not only stuck with tile cement, it’s mechanically fixed too. (Screwed down)
That would eliminate any special preparation techniques that may be required for the particle board.
Whilst a cement board overlay is an inert material, not effected by temperature fluctuations or moisture, the chipboard isn’t as stable.
The ultimate choice for your install would be an inert structural material.
There are two such materials that meet that requirement.
1. GIFA board, a gypsum based material, &
2. 22mm No More Ply. A cement board.
If you were to chose the former, you would need to use their primer for the GIFA board, and follow the instructions carefully.
As gypsum and cement based adhesives are not friends unless primed correctly.
But both are excellent products.