Lossy compression != lossy decompression/restoration. With vanilla JPEG, you should be able to redo the DCT and find the quantized values exactly as they were,

Lossy compression != lossy decompression/restoration. With vanilla JPEG, you should be able to redo the DCT and find the quantized values exactly as they were, which means that you could losslessly reverse the JPEG compression not in the sense you get a compressed version that decompresses to the same lossy reconstruction. With deblocking filters in MPEG2 and later, this is not necessarily the case, because you try to smooth things over in decompression and can't reconstruct the compressed version either.