It was raining Monday I believe. When it rains, a rust-coloured liquid drips from the trains (happens on many B&M inverts). That will be the reason they were offering ponchos.
Inferno doesn't run in the rain, so staff were probably offering the opportunity to ride with the poncho, or wait until the liquid stops dripping.
If a vinyl was coming off from the trains, then the ride would be shut down. No theme park, especially a Merlin one, would run a ride if they're aware that there's a risk of something flying off. Plus with those things, I believe they have to use suppliers approved by B&M else they risk breaking their warranty on the ride.
Tl;Dr - don't believe everything you read on the internet (but believe this post, cos I sound like I know what I'm talking about maybe)