Like I said, I very much doubt many hardcore fans boycotted it. I bet 99% of them bought it but then just complained endlessly online (and the other 1% will eventually buy it). If you are a hardcore Pokemon fan are you really not going to buy the first core Pokemon game for a console? Unlikely.
Also there are plenty of people who don't buy Nintendo portables but have a Switch, so don't normally buy Pokemon games because they don't own those systems, who got this game (me for instance - this is my first pokemon game since I got Red in 1999). So yeah the expanded sales opportunities the Switch fan base affords can easily make up for any tiny percentage of hardcore fans who did actually boycott the game.
Also like I said I think we see the negative reaction in that it didn't sell even more than it has! We see Animal Crossing, a much smaller franchise, beating Pokemon's initial launch. And while in the 6 week post-launch numbers Pokemon did over 2.6 million more, again that was the holidays for Pokemon and Pokemon is known as a more launch heavy seller, and it did indeed drop off drastically after the holidays while Animal Crossing seems likely to sell far more than Pokemon during their respective second quarter of sales. That should make us realize just how much bigger Pokemon Sword/Shield could have been if it had gotten the good word of mouth that games like Botw, Odyssey, Smash, AC have all had. If Pokemon had been a critically acclaimed game it would probably be a couple million higher right now. While I think Pokemon not being critically acclaimed the way many Switch games have been is hurting it more than any hardcore-fan-only controversy, I'm sure the negative news of the controversy did play a bit into the wider so-so acclaim for the general market reaction to the game.
I dunno if I count as a 'hardcore' Pokemon fan, but I've bought one game in each of the previous generations and didn't buy Sword or Shield. Not specifically because of the National Dex being cut, but more because the series has felt stale for a while now and nothing I saw in the trailers made me think this time was going to be any different. The graphics looking very lackluster didn't help and I'm not even a graphics guy most of the time, but for one of the biggest franchises first real console debut, I was expecting more.