I am not an audio technician so I can only speak from what I have read.
The 0 dB refers to audio levels but the volume filter is only about increasing the volume and is not about the audio levels. They use the same unit but they are not about the same. If you use the volume filter you are not able to assess the audio level. The audio level is also described with dBFS which is better to distinguish from just gain in dB.
If you want to measure the dBFS and other loudness related statistics just use View > Scopes > Audio Loudness. That should give you more insight. I would recommend you search for guides about LUFS if you want to achieve the optimal loudness for YouTube.
By the way, you can right click on videos in YouTube and press on Stats for Nerds for more information about the audio normalisation. If the values are negative your content is not changed, if it is positive it will have a decreased loudness (http://productionadvice.co.uk/stats-for-nerds/).
(You could listen to one of my videos and tell me if they are loud enough. I could post the values to the video if you want.)