I honestly think it's a load of bollocks - sperm meets egg, creates blastocyst, which attempts to attach to womb lining and then the cells multiply. None of that happens due to anything in your mind - same as how your heart knows how to beat, your lungs know how to breathe, and your stomach knows how to digest regardless of what's going on in your mind. It really helps me to remember it's a bodily function same as any other.
If I was to worry that perhaps my oesophagus will not work properly, it would have no effect on whether it would actually work.
Now, it is true that periods of stress can delay ovulation, but the levels of stress would generally be much worse than the worries of TTC - and that's not me downplaying the stress of TTC, merely commenting that it refers more usually to those suffering from extreme grief, often coupled with unplanned weight loss or gain (which can be the real reason for delays in ovulation).
When I find myself concerned that my mindset is causing a problem, I remember that conception occurs all over the world in all sorts of stressful situations - incarcerated women, women in war zones, women in domestic abuse situations. Some of those women likely didn't want pregnancies and their mindset didn't matter unfortunately, and some probably desperately did want a baby, and their stressful circumstances didn't stop it happening .