It is a general statement, there will obviously be exceptions.
If you know something the market doesn't then yes you should invest but generally you are better buying shares with positive momentum than negative.
I wouldn't be buying until momentum changed.
Following the 200 day rule you would have sold last August at 363p, having made a 134% profit.
This would follow on from the 779% profit you would have made in 2018 following the same rule.
HALIFACTS
"OWLS, just take a good look at that simple rule again for us. I think the reason for bigpunt's comment may become clearer to you. :-)"
No, not any clearer I'm afraid.
By following the 200 day rule you would have bought in April 21 at 238 and sold in March 22 at 317, a nice 36% profit but more importantly you would then have avoided the subsequent painful 40% drop.