This would not surprise anyone who's flown with TCAS, or a radar information service, because of the number of times you're warned of aircraft that you can't see - even though you've been informed where to look.
I agree, TCAS makes you more concerned about maintaining a good lookout because you realise how many you probably failed to see before you had use of it. Conversely, pilots
not having had use of TCAS or similar systems probably have a false idea of how busy the sky actually is. Even in 8/8 blue conditions the human eye is not good at spotting fast moving targets which occupy the same part of the retina - i.e. are on a collision course.