”They canreplace the role six months later.”
Where did you hear that?! There is no such rule. For a redundancy dismissal to be fair and lawful, it needs to have been a genuine redundancy situation at the time the decision was made (and still be a genuine redundancy situation by the time employment ends).
There is absolutely no rule saying that if business needs change again, they cannot recruit within a set period, or that the start date of anyone new needs to be a set period after the termination of the redundant person.
Obviously if they recruit quickly, however, it does undermine the initial argument that there was a genuine redundancy situation, so to re recruit without the original post holder bringing a successful claim, a business would really need to show that there had been a genuine and significant change that could not have been foreseen.
Because the time period for bringing an unfair dismissal claim is three months, in practice, even if it wasn’t a genuine redundancy situation, once that time has passed, the business can do whatever it likes. It is prudent to wait a little longer in case there are any circumstances which might cause the time limit to be extended or to make sure there’s nothing drifting around ACAS.