This is straight up not correct:
https://bitcoin.org/en/developer-guide#p2p-network
https://en.bitcoin.it/wiki/Full_node#Economic_strength
https://en.bitcoin.it/wiki/Myths#Miners.2C_developers_or_som...
The mining process makes it expensive to lie to the network, but if no one is checking for lies it could still be lied to. Full nodes check that mining nodes aren't including invalid transactions. The requirement to submit a valid hash with your mined block means that the network can't be spammed into oblivion. Check the hash first (cheap), and if the hash passes, check /everything/ in the block to make sure it's all valid (expensive). If the block contains only valid transactions accept it, federate it to your peers, and thus give the miner lots of money. If the block contains any invalid transactions, reject it, thereby levying on the miner an opportunity cost for doing the wrong thing.
Removing the second step of validation would mean that you could attack the network with considerably less than 51% hash power. You could buy 1% of the hashpower and submit fraudulent blocks 1% of the time on average. Note that submitting fraudulent blocks is not the same kind of attack as a 51% attack.
Anyway, you see this "full nodes don't validate the blockchain" nonsense a lot from altcoins that think they've found a neat-o way to get faster/cheaper transactions. They usually have, but they invariably have to lose the trustless nature of the system to do it, at which point you'd be better of using a regulated bank. It's a big red flag that the developer of a coin doesn't actually understand the technology they are using.