Processioning Rats is rarely a good idea, as it gives you two more rats, and only one 5 cost card.
It actually only gives 1 Rats, because you trash the one you Procession. So Trash 2 cards, Gain a Rats, Gain a $5 card, +1 card from trashing Rats.
either way, you've got one more rats than you started with. the only way to trash them is altar or farmland, neither of which is really ideal.
it's something like playing two rats, a village (from procession), a lab (on-trash benefit), gain a $5.
This is much better than just rats. the requirement of matching your procession with rats is negligible if you have a lot of rats.
Here it works because Ruins are gone, and you can empty Rats and Markets very fast for a surprise 3-pile ending, plus the ruins give you lots of things to convert into Rats. Now that you've got a lot of control over the end of the game, your ability to get points is rather limited, your best source of points is farmland, which is just one point better than duchy.
So I'd say it's not to be attempted without awareness of what made it work here.