620.91(B) talks about other building loads being auotmatic, does that mean auto transfer switching, to a panel to absorb this regenerated power?
IHMO it means just what it says, paraphrased as:
If there are normal building loads which you can be sure will be turned on whenever the elevator (or other regenerative load) is operating and which added together are as large as or larger than the maximum regenerated power, then you can rely on them to absorb that power.
But you must automatically or manually transfer that group of loads to the same(!) generator or paralleled generator system at the same time that you transfer the elevator to the generator. There cannot be any way that the elevator could be transferred without sufficient other loads being transferred too.
To me that means that if there is more than one automatic transfer switch involved, the elevator switch must not transfer unless the other load switch does too. In the case of a manual transfer switch (unlikely or maybe not permitted), the same switch must make both transfers.
And as a design concern, I would want to be assured that nobody would go around turning off all of the lights, etc. while leaving the elevator running!
As a side note, if no normal loads are sufficient, then some other power absorbing load must be arranged. And if you do not want the generator powering that load full time, you would need some arrangement that switches that load on whenever a power meter (NOT just an ammeter) connected to the elevator feed determines that regenerative power is being produced.