The only way I could see a possible case here is if they asked you about conviction instead of arrest. Some employers don't ask about arrest; they're only interested in conviction because lots of people get arrested. If you dodged the question of conviction you might have something?
Always disclose the arrest, if asked, then explain the circumstances and that you were not convicted. I'm a little surprised that you consented to do PTI based on your details, but what's done is done. Unfortunately, some employers have a brain dead zero tolerance policy. In this case, you may have found out prior to an offer being made if you had disclosed.
Anybody can be arrested; it's no fun, but the police arrest the wrong people more often than you know. Sometimes the police arrest everyone in site until they can nail down the right person.