Basically, it appears the first launch of a new install got confused by existing logins.
Ensure you are not logged into zoom elsewhere. Perhaps, still logged into a previous version but not tested; the zoom installer on Mac asked me to quit zoom to continue installing on Mac. This logging out was also essential for first launch of my Elementary OS install.
After much trial and error attempting to upgrade a previously-working zoom, using apt, gdebi, and eddy, the best I achieved was the zoom icon under Applications/Internet; launching put the zoom icon in the dock for about 30s, then gone without an interface.
I was searching for advice on computer #2 while still logged into zoom client. When I logged out on #2, #1 launch coughed up a dialogue warning me I was logged into another client. I quit #2, relaunched #1, and now #1 appears to work as expected. I found this odd enough to mention but I don't have chutzpah points to comment. The following dialogue appears the same for Elementary and Mac.