They really did manage to benchmaxx test262 and beat everyone at it. In my testing (all engines with experimental flags in same conditions on full test262):

  99.5 jsse
  99.1 v8
  99.0 spidermonkey
  98.1 libjs
  97.4 escargot
  97.3 jint
  96.4 boa
  95.0 graaljs
  93.2 kiesel
  92.1 jsc
  82.8 quickjs
  82.5 quickjs-ng
  82.1 xs
  80.1 brimstone
  77.7 nova
  74.6 jerryscript
  66.5 sobek
  65.5 goja