alternative full node implementation usually just follow default Bitcoin Core behavior
True, but in this case, Knots has supported full RBF for a long time now, and if a Core node really wanted to run full RBF they could do that too with very minimal code changes. Relying on zero confirmation transactions has always had an associated risk, and was always dependent on nodes not implementing full RBF, which they could have done at any time.
Well now they have at least another year to decommission their zero-conf transaction scanning, and move to Lightning Network instead. All the major payment gateways support it, so I see no reason why an in-house solution shouldn't be able to do the same.