There is really no reason to use a join, the table function will join the data on the matching field:
(index="xyz" RuleModule="BizRule") OR (index="xyz" client.xyz ) | rename timeTook as "Rule Response Time"| rename timeTook as procTime | table dateTime, policyNo, "Rule Response Time", procTime
This assumes there is a one-to-one match based on policy number. If you expect there to be multiple events per policy number you can use stats:
(index="xyz" RuleModule="BizRule") OR (index="xyz" client.xyz ) | rename timeTook as "Rule Response Time"| rename timeTook as procTime | stats max(dateTime), avg("Rule Response Time"), avg(procTime) by policyNo
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