I don’t know how this idea possibly gained traction. If you asked me to come up with a short list in 2010 of who the next Supreme Court appointments of a Republican President would be, all three would have been in the list. Gorsuch and Kavanaugh were well known “feeder” judges in the Courts of Appeals, like Judge Garland is and Justice Ginsburg and Justice Souter were before their appointment. Barrett was a prominent law professor. The appointments were completely unsurprising.
It's just a talking point that gets repeated by folks that don't follow the legal profession in general or the federal courts more specifically. It's completely ridiculous to claim that Barrett is a "conservative sockpuppet" as some commenter below us makes. She's been a law professor most of her life. But everyone plays dirty: Garland should've been on SCOTUS (his work on the OKC bombing prosecution speaks for itself, and is not to mention patriotic); it's a shame that Congress blocked that vote, but politics is politics.