"Unreasonable" here means that it works way better than it should for something so simple. It's a somewhat common phrasing. Example: "The Unreasonable Effectiveness of Data" https://static.googleusercontent.com/media/research.google.c...

It's a meme, like "_____ Considered Harmful". That one started with "GOTO Considered Harmful". This one started with: https://en.wikipedia.org/wiki/The_Unreasonable_Effectiveness...

It’s “unreasonably effective”, meaning that a cost/benefit analysis is very clearly tilted towards benefit.