When The Clash were making records the definition of punk was up for grabs: to say that 'The Clash' is their only punk album is to mistake distorted guitars for the definition of the style, but there are a lot of bands from the last 30-35 years that have taken a far more rockabilly influenced approach to punk. It's just that with the success of hardcore it's easy for people to think that's the only punk bloodline. I also know a lot of punk and hardcore bands that are prone to outbreaks of reggae and dub, that's a long standing relationship in the UK.
Is Sandinista an album of punk rock songs? No, obviously not. It's an album of probing, creative experimentation, and given the band's background in the punk scene, one of the most courageous artistic statements of the late C.20. I'd sooner listen to someone trying to find a new path in music than someone retreading the sounds of the past: nostalgia peddling conservatism is definitely what punk is NOT, although a lot of people pursue just that agenda nowadays using punk rock sounds.