Because the price you can pay today is lower than the price someone else happened to pay a few days ago. It's purely psychological.
Most people who claim BTC is discounted now at $3500 probably also believe that BTC is a long-term good "investment", which would imply that they also think it's "discounted" at $5000.
I guess this is obvious to all, but there are no well-understood "fundamentals" yet for BTC.
The most charitable thing you can say is that buying at $3500 now is a better deal than buying at $3500 previously, given it's now been revealed that someone was willing to pay $5000 at one point.
Why is it discounted? Did their PE drop or what? I mean, why????