Worth noting that this is just for US and for prepaid SIMs, from their paper “We examined the types of authentication mechanisms in place for such requests at 5 U.S. prepaid carriers—–AT&T, T-Mobile, Tracfone, US Mobile, and Verizon Wireless”.

It doesn’t mean that for the rest of the world SMS 2FA is completely secure, it’s just a lot more difficult (or impractical/impossible) to do a SIM swap so easily. As mentioned in another comment below, SS7 vulnerabilities are another attack vector, globally available and without requiring a SIM swap.