It's a cracking tool. If you never needed to crack NTLM passwords or bruteforce windows shared folder passwords over dialup, you might not have heard about it in the casual technology news.
What do you mean by "that's really on you"? I'd normally interpret it as something like... "this is a state of affairs that would be different if you'd acted differently, and you knew or could have been able to know this in advance". Along those lines, anyway. But not having heard about a tool doesn't really seem to fit that.
At some point, a tool is so ubiquitous that it's just odd to not have encountered it. You don't see many accountants that haven't heard of Excel, webdevs that haven't heard of Apache, construction workers that haven't heard of a hammer, or cybersec workers who haven't heard of L0phtCrack.
It means it's their fault because they clearly were not paying attention or their memory has failed them.
L0phtCrack has been decreasingly relevant in the past 10 years or so -- it wasn't available for awhile and some free tools are similar so you were basically buying the rainbow tables -- but if you were in security in the Windows 2000 or Windows XP era, you know of this tool. There was a lot of discussion for years around and about password crackers after rainbow tables became a thing.
It's not like not knowing what Wireshark or nmap is, but it is like saying that you've never even heard of Kismet or John the Ripper. Or like being a DBA for decades that never heard of Informix. Or a programmer for "decades" that has never even heard of Delphi. Like what were you doing in the early 2000s to have completely missed the death of Borland and Pascal and the popular variants? These are big enough events in the industry that if you're in it you're going to be aware of it.
This is assuming that everybody has an interest in cybersecurity. I can come up with equally well known (in specific circles) tools that you may not have heard of.
