Laying Timber Flooring

We’re looking at changing the carpet on our first floor to solid timber (spotted gum).

We’ve received a few quotes. One place suggest plywood be laid on top of the yellow tongue and then lay the solid timber on top of the plywood.

Another place said they can just lay the timber on top of the yellow tongue.

All places will glue and secret nail the timber down.

My question is whether the timber can be laid directly on top of the yellow tongue, or do we need to have the plywood first?

We built the house back in 2014 so the subfloor is fairly new.

Thanks in advance.