The SRFI document says:

|~x week number of year with Monday as first day of week (00...53)

but the reference implementation does:

(cons #\x (lambda (date pad-with port)
	       (display (date->string date tm:locale-short-date-format) port)))

it is probably a copy and paste error while composing
the document. What the implementation does looks
like what "strftime()" or GNU date do, which almost
all the other escape sequences conform to.
--
Do, or do not. There is no 'try'.
Jedi Master Yoda