It is a general popular error to assume that those who pity the people the most are those who care most for their well-being.
The English translation of “plaindre” should be “complain”. To translate it as “pity” in this sentence is wrong.
I think it might be correct as is? “plaindre” means to pity. (“To complain” would be “se plaindre”)
Reading the French sentence, I think what it’s trying to say is something along the line of “Just because someone has a lot of pity on others doesn’t mean they care about those people’s well-being”. The point being - “pitying someone doesn’t mean actually caring about that person” If this is correct, then “complain” would not work here.