How was Ben Rickert able to secure an ISDA agreement?

I work with ISDAs at a large investment bank every day (since 2014, so granted, after the time period of this movie) but there is no regulatory capital requirement to enter into an ISDA. I believe the $1.5BN referred to in the movie is just the bank’s internal policy (either from a risk standpoint or just based on smaller clients not being worth their time or balance sheet usage). Banks make exceptions to their internal policies (not regulator requirements) for relationship reasons all the time and I believe Ben was enlisted to convince the bank to do so in this case.

Rickert has connections. Moreover, he's considered to be exceedingly good at spotting valuable opportunities, and people are going to take his advice that into account.

When Rickert speaks to the banks and tells them that giving them an ISDA is going to work out in the banks' favor, they're going to be much more inclined to deal with that small hedge fund they would have otherwise ignored.

It is clear from the context given in the movie that Rickert had previously been a major trader. This strongly implies he held an ISDA before his "retirement" from the business. So it is quite possible that he had a pre-existing "seat at the table" and didn't need to re-validate to get a new one.

It is also true, as other answers mention, that the stated requirement is an internal policy matter and not a legal standard. So it is also certainly true that Rickert's history would have given him the connection he needs to get access to the platforms for high-value trading. But, given the vague status of the ISDA threshold, the idea that once you have met the threshold, you retain your status, is simpler.