The argument went approximately:

• 1. We have 5-10 manuscript quotations of Galatians 2:1 with the word “again” missing. The again obviously is referring to the trip in Galatians 1:18-24.
• 2. Galatians 1:18-24 is not attested in Marcion’s Apostolikon. Most notably, Tertulian in arguing against Marcion goes into great detail describing the contents of Galatians 1 and 2 and seems to completely skip over Galatians 1:18-24 despite that it seems extremely likely that he would have wanted to comment on it. In particular it is hard to imagine how Tertulian would have skipped over the mention of James the Lord’s brother in Galatians 1:19 and not come up with some way to work that into an argument against Marcion.
• 3. The message of the surrounding text flows well and arguably better with the first trip to Jerusalem removed. One of Paul’s major points that he is trying to impress on his audience is that the gospel he preached comes from no man. If you remove the paragraph, then Paul is saying that he spent 14 years preaching his gospel before ever going to meet those who were esteemed as pillars before him.
• 4. Having spent a notable amount of time with Cephas a decade before the second trip, it’s a little weird that Paul states that one of the core reasons he went to Jerusalem for the second trip was to confirm that he had not been running in vain and that his gospel was true and matched what those before him were preaching. Obviously if he spent a bunch of time hanging out with Cephas, he should already know before the second trip whether his gospel is in line with Cephas and associates.
• 5. This paragraph is easy to view as an attempt by later orthodox Christians to subordinate Paul to Cephas, so we have a clear potential motive for aln interpolator.
• 6. Stuff I’m not as clear on: there are a few weird Greek grammatical things about the passage.
• 7. The claim “I swear I’m not lying” is a classic thing a liar or someone concerned about potential accusations of lying would say. Paul is boldly making all sorts of claims in Galatians 1 with great confidence, so it’s a little weird that he would go out of his way to defend this particular claim. Why would Paul be concerned about someone calling him out on his claim to have met only Cephas and James on the trip? Why would he think anyone a thousand miles away in Galatia 20 years later in the 50s would have evidence that Paul is lying about this particular claim or be in contact with someone who knew Paul was lying about only meeting Cephas and James on the trip? Paul’s claim that his gospel comes from no man is undermined whether he only met Cephas and James or whether he also met some other apostles.