I agree that YAML block style as on lines 22, 23, 24 should be eliminated (I will probably prevent my framework from parsing this). If you use an unescaped { or } it should signify that you are using Jinja2.
Similarly, I'm no fan of the {% sets %} at the top - it's a code smell.
Apart from those things, though, what you linked to seems easy to read and understand to me.
I'm absolutely positive that Tcl or Guile (or python) would create the potential for bigger messes than what you just linked to under similar circumstances. Simply being turing complete is enough for that.
Similarly, I'm no fan of the {% sets %} at the top - it's a code smell.
Apart from those things, though, what you linked to seems easy to read and understand to me.
I'm absolutely positive that Tcl or Guile (or python) would create the potential for bigger messes than what you just linked to under similar circumstances. Simply being turing complete is enough for that.