Stop the Madness
Violating boundaries and cheating are not characteristics of a healthy, consensual polyamorous relationship. Just because a monogamous person navigated infidelity does not mean that this is the "gateway drug" to polyamory. Healthy polyamorous relationships do not mimic unethical monogamous ones. Comparing the two is an insult to the polyamorous community and rooted in ignorance and bigotry.
People Lie
Being a habitual cheater in a monogamous relationship does not mean you should explore polyamory or are polyamorous. If anything, it means you need to process your inability to respect relationship boundaries. Adding more relationships won't fix that. Polyamory is used as a trump card by some to minimize the severity of their cheating. Cheaters who are polyamorous AND monogamous may do this.
Polyamory Can Be Weaponized
For manipulative cheaters, polyamory can be weaponized as a threat and as something integral to their personality. It can be a very successful tactic when used as a threat to seek other partners. Instead of realizing they are being manipulated, some people dating a monogamous cheater may be talked out of breaking up out of fear of being left. Some may feel inclined to support their partner's supposed polyamorous identity, hoping it would subside the cheating. They may see cheating as something their partner can't help. Once again, people lie. Cheating is not limited to one lovestyle orientation, nor does polyamory solve it.