It’s not a myth, there is a quality difference when you work with Long GOP material vs. getting out of that format into ProRes or DVCPro HD.
If you capture direct to ProRes via baseband (Kona, Decklink, MXO2 etc) then it’s probable you may see some qualitative advantage due to the higher quality of the dedicated decompression hardware in the playback device versus software only decompression, but if you ingest native and/or transcode in software then it surely won’t make a jot of difference whether you transcode to ProRes at the beginning or at the end of the workflow.
There is of course no argument that working in an intraframe intermediate codec is always going to provide better overall performance than working in an interframe codec (always assuming one has a storage system with the speed and capacity necessary to deal with the higher data rates involved).