I'm not sure why Guido's preferences would be relevant.  f-strings support expressions, := is a valid expression, f-strings therefore must support it. 
 f-strings expressions are not top-level statements and therefore will not require parentheses around :=.

There appears to be some confusion around f-strings' use of : to delimit a "format specification".  Supporting := won't break format specifications, although it will require some intelligence--if you see a :, you must examine the next character to know whether it's := or a format specification.  It is not a breaking change.