Of course i understand running full nodes secures network as a whole.
Why would this be the case?
Specifically, consider a network with 1000 mining full nodes and 1000 wallets. Now add a non-mining full node. Is the network somehow more secure?
The full node is a checkpoint on your own driveway. It doesn't secure the rest of the network in any way. And as you explain, it doesn't really make you more secure than SPV due to withholding/releasing attacks being much more effective than any invalid block attack.