Ideas and suggestions.

Then it's unclear why steel golems were introduced at all, if limiting the number of rounds to 100 also solved the problem with factories.

Since they were introduced, it means it didn't solve the problem. Kal'kh could capture even large factories faster than 100 rounds with mages. Tactics with pinning ranged units in a corner (cyclops, mages, lizards, etc.), blocking them with single units, and using a fast unit to attack were also used.
The new factory can still capture a tower using the old tactic, but it requires precise actions, calculating the number of golems, sacrificing the golem stack, using mana for haste, and making 4 constructions to prepare for the capture.
Some Lorelei in a dungeon can also capture it, most likely, but only small factories without upgrades.