Yeah baby boomers are a generation, but not all baby boomers have a boomer mentality. I know some Gen X, some millennials and some zoomers that do too.
I think it's the difference between punching down and punching up. Boomers in general have far more money and power than the people using the term. So complaining that it's ageist, and comparing it to a homophobic slur will get you about as much traction as white people being offended by cracker, and comparing it to slurs used against African Americans.
a mindset from a different time has little to do with age and more to do with the environment they grew up in. If a baby boomer time traveled from when they were 20 to now they would still be a boomer.