title_body: How can I improve this Chemex cold brew method?
upvoted_answer: What you're talking about is the "Japanese iced coffee" method, which is hot-water brew directly over ice. In contrast, cold-brew is using cold water in contact with the grounds. See techniques at other questions tagged as cold-brew.
  
  This article from Counter Culture, also referenced in the article you linked, suggest Japanese iced as an alternative to cold brew. The articles suggest that directly brewing hot coffee over ice retains more of the volatile flavour compounds.
  
  If you have had true cold-brew before, most methods use several times as much coffee grounds to brew as with other methods (some then dilute this with, e.g., water or milk). For example, normalized to a 200mL "cup" (~6.5 fl.oz.) of brewed coffee: Blue Bottle suggests about 3x more coffee for cold brew (45g per 200mL) than for drip (14g per 200mL). If you're coming from that strength, it's no wonder you're finding it watery. 
  
  To your first question: If you want a stronger brew, why not simply use more coffee grounds? Your amount (21g grounds to 300g water+ice is about 14g/200mL cup) is already at the higher end of the spectrum for drip coffee; the industry standard is closer to 7g per cup/dose. But if you want it stronger, use more beans!
  
  On your other question: this temperature change shouldn't be an issue. You said you're using a Chemex (e.g., classic 3-cup) -- it is made from Pyrex brand borosilicate glass, which has very good thermal shock properties. Even a lesser glass (e.g., non-tempered soda-lime glass) shouldn't be a problem at these temperatures (slowly dripping boiling water onto ice). (However, for example, ice into glass at oven temperatures could certainly be a problem).