You can find a good article on metabolism here:
https://www.vox.com/2018/9/4/17486110/metabolism-diet-fast-weight-loss
"Metabolism" is one of those terms thrown around recklessly without most people understanding what it means. There are lots of products, books, posts, diets, systems, etc., that claim they'll help people lose weight by "speeding up" or "boosting" metabolism. Most of this is nonsense (often from people who should know better). Any "boost" is minuscule and transitory.
People can sometimes blame a slower metabolism for extra caloric intake. We're really bad at estimating how much we eat and tend to overlook lots of little things (like sauces) that add up and lead to increased body fat. It's not that we're lying about it; it's just one of those human foibles.
As the author of the article found, "slow metabolism" is rarely a thing. Yes, it does slow down as we get older (which surprises no one over 40). And dieting--particularly extreme dieting--can slow it down too. But while such things can make it more challenging to shed unwanted pounds, it's not even close to impossible (although it can seem that way).