That's what I'm hoping, that the message is just wrong. However:you're inactive but you won't get decay if you're in bronze/silver/gold. if you're inactive in plat or higher you will decay.
http://www.reddit.com/r/leagueofleg...just_got_demoted_for_inactivity_but_ive_been/
Someone replied to this thread a month ago and said they had an alt decay. I'm afraid to play any games because I am at 0 LP and just need another week for the gold rewards, so I'll just trust to Riot's announcement. Unless that other person's post was right before Riot made the change.