Sometimes it is, but I think more often the road to hell is paved with bad and/or selfish intentions. For instance, the Nazis weren't sober-minded radical altruists who begrudgingly came to the conclusion that they had to do what they did for the greater good – they were instead following an ideology which was racist and evil, through and through. I'm also pretty sure that most people who participated in the slave trade weren't doing so with "good intentions", but instead simply to make a profit.
 

OTOH, I think the claim does apply, more-or-less, specifically to socialist movements, and I think it's important that other consequentialist ideologies maintain significantly better epistemic hygiene and course-correction mechanisms than socialist movements typically have. But I'd also point out that global movements for democracy, slavery abolition, and anti-colonialism have also been done with good intentions, so "doing things with good intentions" has a track record of doing some really good things as well, and I think on net is a lot better than alternatives.

Here's my stab at it: Long before the internet, this folk wisdom encapsulates the notion that: A. It's not enough to have an intention without acting on it and/or B. It's often hard to predict the long-term implications of an action. Famous examples: the inventor of the plastic bag was trying to solve the problem of waste from paper bags; Alfred Nobel's dynamite was created as a safe way to blast rocks.

It says something about accountability and the importance of feedback, that is, as consequences accumulate, feedback about them is fairly important. People recognize ideologies that do not depend on feedback for their claims of good intentions knowing that such ideologies are trojan horses for counterproductive plans, as longtermism appears to be.