I do agree that buying ETFs is "cleaner" for all sorts of reasons. The certification process for bonds vs ETFs is slightly inconsistent but given that they're not going to participate in the create/redeem process, I don't have too much an issue with it. If they were buying ETFs, redeeming them, and then actively managing the bonds from there, then it would be a little more murky. Besides the fallen angel ETFs, none of the ETFs they've bought so far (and everything that qualifies as well) does not have any real issuer concentration that you could argue tremendously favors any particular issuer beyond reflecting the actual bond market.
To further expand on the overall topic, if all the Fed did was buy ETFs, there's a real risk that would not be enough. During any crisis, in order to be effective, central banks need to make big moves. To use a crude metaphor, individual bullets fired separately don't have nearly as much impact as a single bazooka round and sometimes you only have a chance for one shot. Bond markets aren't as efficient as equities and without secondary purchases, there could have been a real chance that you had ETFs more or less stabilized but several underlying issuer/sectors/whatever aggregate group continue to be dislocated. For another crude metaphor, sometimes the tail (ETFs) wags the dog on this stuff and vice versa. In order to have functioning markets, you need complete control over the entire animal which would be primary issuance, secondary bonds, and one of the more liquid proxies.