[fulafel]

That's not quite how SMT works. Half of the core is not sitting idle when running a 1 thread workload on a 2 thread core. Rather the core will be working at mostly full tilt.

Also if only one core is active, it can be put into a higher than normal power regime.

[detaro]

As far as I know, 100 % in the Activity Manager is one hyperthread fully busy, regardless of clock. It doesn't seem too unlikely that Zoom takes that from the equivalent API, divides it by the number of seen virtual cores to convert it to some measure of "overall CPU load", and reports it as usage. It's not a good measure, but an explanation. (And as you point out boosting happens, so the "55%" reported by Activity Manager is already not all that good)