The authors go to great lengths to insinuate, but not state outright, that these firms are replacing those laid-off workers with H1-B visa holders. They do this

The authors go to great lengths to insinuate, but not state outright, that these firms are replacing those laid-off workers with H1-B visa holders. They do this because the data is insufficient to draw said conclusion. This data does exist for at least the top two firms discussed, because both Google and Meta are subject to the WARN Act and must provide public notice of the actual count of terminated employees broken out by job title, and the USCIS by necessity holds these records per-employee and -employer. The argument for H-1B reform is compelling enough on its own merits, so it's unfortunate that "firm X laid off M people and hired Y people through H-1B" point is being used here. After all, these same firms hired more non-H-1B than they did H-1B employees during the same period.