Alucard is popular, and I'm sure he especially gained more traction because of the Castlevania Netflix show that first aired in 2017. Season 2 of that show aired on October 26, 2018, two months and 18 days after Simon and Richter were revealed to be playable.
He was definitely the most popular Castlevania character before the show.
Being the protagonist of Symphony of the Night, the most popular entry, helps a lot, but he also has multiple appearances across the series unlike most Belmonts. Non-Castlevania fans will definitely know about the Belmonts and the Vampire Killer, but I feel they might not know nothing about a specific Belmont (be it Simon, Richter, Julius or another one), but they will probably know about Alucard, specially now after the Netflix show. At least that's my perspective after all these years. Before I played my first Castlevania game years and years ago I was already familiar with Alucard, but I didn't either know or remember any specific Belmont.
As for his words when picking a Castlevania rep, I feel he was talking about not only Castlevania fans but also people in general, people who never played Castlevania I mean. And with fans, I understand that he means "Smash fans". Simon was the one Smash fans pushed the most. The Belmonts are also more synonymous with Nintendo consoles, with Super Castlevania IV being a defining title of the SNES.
All of that was in a period where the Smash community felt Nintendo connections were a big factor for third parties. I'm wondering... would we had got Alucard instead of Simon/Richter if that idea didn't exist before? Now, almost any character feels somewhat realizable regardless of being enough Nintendo or not, which wasn't really the perspective fans had years ago. If we imagine a scenario where Castlevania wasn't added to Ultimate's base game but ended having it's chance for, let's say, Fighter Pass 2 or the next Smash game, I feel that Alucard would have been in a even better position, because I'd assume fans would start pushing more for him and not just Simon.