This idea that they are not competing doesn't make sense.
You're right, because they are indirectly competing.
XBO/PS4 is direct competition because the better one sells the worse the other will sell.
Switch/PS4 or XBO is indirect competition because a massively successful switch won't have a big negative impact on a PlayStation or Xbox console.
PlayStation and Xbox are both fighting over the same scraps, and a person is much less likely to own both a PlayStation and Xbox than both a PS/XB and a Switch.
Though even if you reject the terminology, it's indisputable that PlayStation/Xbox have many more similarities and are much closer linked in competition than the Switch.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)
Switch lite really screwed up my Switch prediction this year! Looks like it's going to be about 19.5m-20m for the Switch in 2019, 14.3m for PS4.