Debilinear() is not meant for that, debilinear is for undoing a previous bilinear upscale. Google tells me some people use it on the chroma; they are assuming the studio used a bilinear resize to get the 4:2:0 chroma from a 4:2:2 or 4:4:4 master but that seems crazy to me, especially with a DVD source.
NNEDI3 can make up details, sort of, but they aren't the real details. If I wanted to make upscaled DVD rips I would use NNEDI3_rpow2() followed by a downscale to the desired resolution using my favorite linear light resizer (I like Dither_resize16nr from cretindesalpes'
dither tools). The inverted kernels (debilinear and similar) are not appropriate for this step as the output of NNEDI is not from that kernel.