I feel like it's a misunderstanding of a few variables as well as possibly older FPS configs not working as well:
First, I think the main premise of this was because people associate DX8 with lower quality, while it is much more than that. It changes the entire rendering pipeline to something which doesn't work as well on newer GPUs. The speed boost that people see at all is simply because DX8 supports less features, and thus renders less, but it renders less at a lower efficiency.
Furthermore, many configs in the past did not properly save changes to TF2's video config, which caused the game to reload many times on startup, causing graphics systems to initialize in pathways which could have resulted in glitches and lower performance. mastercomfig saves the graphics options that you select to the video config, so that these reloads are avoided. To completely ensure that this is the case, use modules entirely instead of individual rendering commands (usually prefixed with mat_, also applies to r_rootlod).
Finally, I think that in some cases, people found that their game ran better without a config, and I don't want to disparage the work other people have done, but perhaps people associated a config with only changing graphics quality, while it does much more than that, and thus, can affect the game's performance beyond lower quality graphics.
All of these things can combine together for a community to eventually come to the conclusion that lower graphics are bad for performance on some systems, even though the reality was slightly different. These games of telephone are very common in this community as well as any other community. As information is passed along and reworded, re-understood and re-contextualized, it can gain new meanings which point to new conclusions.
In my time looking at how people have benchmarked each of my presets, I have found that each preset scales in respect to FPS as you would expect from low quality to ultra quality, and it is close to linear (with some softer bumps across the medium presets). So, I don't expect this rumor to do you much good.
I think a good way to think about it is that these sorts of options are rarely part of some zero sum game, where the more quality you stress onto your GPU, the less work your CPU does. Most often, it's a constant amount of CPU work, or more CPU work, in combination with increasing load on your GPU.
There is, however, one case where increasing the quality is a good tradeoff, and that's if your GPU is throttling below its max clock because of power saving measures, since it doesn't realize it needs to render at full capacity for TF2 since it is generally lighter on the GPU than other games. Usually, you can control this in your power options, both in the driver settings (in NVIDIA it is called "prefer maximum performance") and in Windows 10's graphics settings "graphics performance preference". In the vast majority of cases, this will solve the issue, but some cards may still not scale correctly and thus it may be worthwhile to look into the medium high or high presets.