This was a joke, The note was left by Liam Byrne to the incoming treasury secretary. The tradition is to leave a handover note with some helpful advice for your successor.
A similar message was left by a previous Tory to Labour Treasury Secretary of "Good luck mate.. Sorry to leave it all in...
Yeah we are all guilty of confirmation bias.
The narrative that the Tories are "good" with the economy and Labour aren't persists.
Ultimately it comes down to ideology though.
If you believe the economy is being managed well because your tax has gone down, but your public services are...
The reason you think this is just political confirmation bias.
I'm guessing your instinct tells you that a party that makes cuts is managing the economy, whereas a party that promotes investment in public services is economically irresponsible.
A lot of people believe Labour ruined the economy...