Avengers: Endgame - 20 Things We Learned From The Director's Commentary

10. The Thanos Pym Particle "Plot Hole" Is Finally Explained

Another dubious plot hole - or rather, something that isn't explained to the audience - is the means through which 2014 Thanos and his army are able to travel to 2023 despite the heavily limited availability of the Pym Particles required to time travel.

Joe Russo confronts this issue in the commentary, adding that the "obvious assumption [is that] Thanos, who has a spaceship that can carry his army throughout the galaxy and conquer planets, can also...add some Pym Particles."

McFeely himself amusingly adds, "Best not to dwell on it", seemingly confirming that it's a "because movie" convenience that audiences just need to accept and move on from.