DC8,
They are talking BS.
You are allowed to use the Speedbrakes from down up to Flight-detent. However, the effectiveness of the speedbrakes is very limited indeed if not being put into flight-detent, especially from 250kts. onwards. Always be gentle with the speedbrakes, deploy and stow slowly and leave your hands on the lever at all times.
Try not to use the Speedbrakes when flaps are deployed, although not forbidden by Seattle, it is aerodynamically not recommended... better put the gear down in this case, in my opinion.