From what I can find, Healthline just summarizes medical info and research. The original claim goes back some ways, but I found a study from 2008 by Ishikawa et al. demonstrated that bee pollen inhibited mast cell degranulation in mice. Also, as stated in the article, a 2021 in vitro study where bee pollen inhibited inflammatory responses after mast cell activation by allergens.
But as stated in both articles, there is a lack of extensive human trials, and more research is needed to confirm these findings. From some other sources I found:
-Center for Research on Ingredient Safety (CRIS): CRIS at Michigan State University states that while bee pollen contains nutrients beneficial to human health, it has not been shown in clinical studies to cure or significantly improve specific medical conditions, including allergies.
So it seems some studies show evidence that suggests that pollen could be an antiallergen, but there aren't enough trials and evidence on humans to say for sure.