Made it work using %X instead for time

timestampformat = '%-m/%-d/%Y, %X.%g %p'

or

CAST(strptime(TimeGeneratedUTC,'%-m/%-d/%Y, %X.%g %p') as TIMESTAMP) TimeGeneratedUTC

Still curious why %-I:%-M:%-S does not work instead of %X:

timestampformat = '%-m/%-d/%Y, %-I:%-M:%-S.%g %p'

(neither in read_csv nor cast)