Wondering at sweet peas resilience in the freeze...

... I have been working from home for a couple of days now, and, when going out to empty the bin, I noticed that my sweetpeas are still alive, despite the freeze. I thought they were supposed to die (after all in my experience they have died for much less before) but no, these ones I planted out late and they can obviously be hardy annuals (they are not the perennial Lathyrus of that I am sure)... will have to investigate more when I have some spare time... anyone knows?