Why would having a larger pool make dating harder? I think the author is confusing the probability of a “stable marriage” with the probability of a relationship . What this says is you’d expect cheating to go up. As Chris Rock says, “you’re only as faithful as your options.”
FWIW, I have found online dating way harder than dating someone I met through friends/work/school. YMMV but my theory is having access to that many more people leads one (including myself) to sometimes think there's someone more attractive/interesting out there instead of giving what's available to them a strong chance.
I have two good friends who have been serial daters for years and they have Seinfeldian reasons as to why they didn't fit with someone. It made me think my God, "if everyone had your standards, literally no one would have a long term relationship." Interestingly, when I saw them date mutual friends back in Uni, the relationships were the longest they've had their entire lives.
I'm convinced that a lot of stable marriages from people I know around me wouldn't have happened if they had tried first meeting online instead of happenstance in IRL situations, because person A wasn't the typical ethnicity that marries person B, or person A is much older than person B and wouldn't have fit in the filter, or person A isn't particularly photogenic, etc.
I'm not trying to paint online dating as a bad thing as there are quite a number of success stories.