People may choose to keep their positives unreported/not know how to report/report incorrectly etc
What would the reason be for them doing this though? If they weren't going to do anything with the positive, why would they test? If they are going to isolate, then they why wouldn't they log it?
This sounds a bit like another "no-one will follow the rules" claim, despite that we've never actually had any evidence other than the vast majority of people do follow whatever the rules are.
Of course it's going to impact the stats, there will be a change, but I don't think it's going to suddenly turn us blind, especially when the majority of cases are not LFD ones currently.