I really don't think it was the cold. I mean, it went down to -2 last night and I have 4 - 8 week old silkies that had no heat lamp at all. They all made it through the night with no issues what-so-ever. She could have just had other issues..? I know my silkies are extremely hardy and very healthy.
I think hardiness may be relative to the environment in which silkies are raised in. Meaning, if 8 week old babies have been outside and gradually gotten used to the diminishing temperatures, they may be fine without a heat lamp. But, if someone has been keeping 8 week olds inside and wants to move them out into a coop...they may need a heat source. I also believe that wind, wind chill factor and dampness all make a difference. I now use AccuWeather which calculates a RealFeel temp based on a variety of factors. The app works well in the summer also, when humidity plays a role.