It means if you add a tip there's a higher probability of them screwing up the delivery, either not finding my flat (apartment) at all or being severely delayed

It means if you add a tip there's a higher probability of them screwing up the delivery, either not finding my flat (apartment) at all or being severely delayed because they sent someone who cannot read the simple English directions. If you do not add a tip then this tends not to happen. It's the inverse of what you'd expec. It may be some side-effect of Deliveroo asking for the tip in advance - one theory is that the delivery agent thinks I'll give a cash in hand tip and therefore tries harder; or it could be something more obscure, eg perhaps the illiterate delivery agents (of which there are many in London now) generally work faster but cope less well with any modest complexity and they are being rewarded by an algorithm that does not take that into account.