

Either he was arrested with no record of the arrest (i.e “disappeared”) which is a new line for the administration to cross, or he disappeared for another reason (by himself, or with help or coercion by a foreign entity like the CCP), and the FBI is investigating.
I hope journalists keep on this because the first option would be a huge problem that everyone needs to know about, but without more information, the second is also a possibility. The CCP is known to have agents in western countries that manipulate and pressure ex-pats to return to China. Keeping an open mind now will also strengthen the argument if evidence for the former comes to light.
Phone number and trust-on-first-use for most people, with out-of-band fingerprint verification for the paranoid. It really depends on the threat model and the security practices/awareness of your colleagues, but a link shared on some social media or lower-security chat network is more vulnerable to a man-in-the-middle attack than a phone number for your average Joe. There are a lot of ways a person could get a manipulated invite link.