This is a different country today, in which it is possible to be convicted other than by a jury of one's peers or a Bench of Magistrates who, being lay, are also one's peers.
Juror intimidation is as old as the hills. Isn't that why we pay the Police? If this sort of measure was not necessary against the Krays, then why is it necessary now? And note how, whereas a foreman of a jury says only "guilty" or "not guilty", the judge in this case took twenty minutes to explain his verdict.
Did these men do it? I expect so. But a jury of their peers has not unanimously found so beyond reasonable doubt. That is the point. If it had done so, then both a very long prison sentence, and a very stringent regime while in prison, would have been wholly appropriate. But it has not done so.