Technically I believe the Pepsid is also an antihistamine if I am not mistaken. They work on different receptors.

I am not certain of the actual science, I have read the hypothesis but don’t understand it enough to explain. If you do a web search on “famotidine and cetirizine for long covid”, you should find a number of sites with various references to this treatment approach. It seems it was initially tried in active Covid patients who were hospitalized and some feel it has benefits for some long covid sufferers, specifically those who may be experiencing a cytokinetic storm/excessive immune or inflammatory response.