According to the stats on the BBC article, Kyrgios has better first serve accuracy than Sabalenka (71% vs 62%). Also, the way he plays, his second serve is often more like a first serve, so the single serve is unlikely to have the effect you suggest. The only thing that might affect his serve is the smaller court to hit into.
Does it give first vs second serve accuracies and speeds? 'More like' - but still likely that both players will have a second serve that is slower and more accurate than their first serve. It will give Sabalenka more chance to get keep the ball in play.
If you've watched mixed doubles this will be clear. When the M player serves to the F opponent, it's least likely to get returned of all the combinations - but if he faults the first attempt, it's more likely she'll get the second serve back.
Even if his second serve was identical to his first, I think many male players would be happy to get 30% of points off Kyrgios on his serve. In reality, he will have and use a slower, safer second serve, and more points will get to the rally stage than would have done without the adjustment. Which is what the adjustment is trying to achieve.
He'll get her serves back much of the time anyway first or second serve, but at least then they're in a rally. I think they think that rallies depend more on skill than raw strength, at least compared to serves. And that the women's game being also more centred on rallies compared to the men's she may have more skill than him. Combined with the different court sizes to counter speed advantage in rallies. If they're both having to run from one corner of the court to the other, she's running less far.
I don't see they've done anything to mitigate his power advantage in rallies though. And of course she'll also be using her safer slower second serve so those will come powering back at her.