It's an installed equipment limit.One of the main reasons I think the 250k is sandbagging is that 250,000 units a year is now kinda niche for Tesla.
250k a year is a vehicle every 2.1 minutes, 24/7/365.
The 9000 ton Gigapress has a dry cycle rate of 1.2 per minute or 50 seconds. That leaves a maximum of 76 seconds to cool (if needed), remove casting, and prep mold for the next shot.
At 90% up time: 1.9 minutes per casting or 64 seconds of overhead
80% up time: 101 seconds, 51 overhead
Current presses are running at around an 80-90 second cycle time.
So they might get more than 250k, but likely less than the next multiple of 50k.