So... the article describes that:

... the simplistic nature of the [Houthi Anti Air] systems also helps them to avoid earlier detection by America’s advanced equipment. “Many of the [SAMs] are also improvised, leveraging non-traditional passive infrared sensors and jury-rigged air-to-air missiles that provide little to no early warning of a threat, let alone an incoming attack,”

and:

but the Houthis claim that the Barq-1 and Barq-2 [Iranian AA missle systems] have maximum ranges of 31 miles and 44 miles and can engage targets at altitudes of 49,000 feet and 65,000 feet, respectively.

with some of these missiles being:

capable of firing Taer variants also reportedly have electro-optical and/or infrared camera to aid in target acquisition, identification, and tracking.”

... So I find it rather odd to describe passive IR guided AA missiles as 'non-traditional'.

I think a better phrase would be 'novel' or 'unaccounted for'.

Passive IR missiles of different exact specifications are... pretty common through the entire history of ... just missiles, in general.

Jet engine exhaust is extremely hot, and it would seem the F35 is not actually as good at masking it as previously thought, probably when its flying away from the missile launcher and is thus showing its big hot ass... if passive IR + electro optical missiles can get this close.

('electro-optical' is a fancy term for basically a visual spectrum camera + computer tracking an identified target... you know, like a snapchat face filter...)