We too had an minimum of 75 for a Friday or Sunday wedding at the venue as Saturday wedding would be 100 minimum. We would have been responsible for paying for the meals of the count being under 75. Luckily, we did find others to invite when declines came through to cover our bases. The vendors were also included separately since we had to pay for their meals, which you have to do the same to consider part of their contracts.
No, it means they want you to pay for 150 minimum. Even if you have 149 people, you're paying for 150 meals. They want a guaranteed amount of business set at a certain amount during peak wedding season.
What you can try to negotiate for is a more expensive meal if you can't hit 150. If they're commanding a minimum, it likely means they have an audience who wants it.
All venues usually have a minimum spend, whether it be head count or cost. This is especially during peak summer wedding season.
You can see if they're willing to adopt a lower head count with a higher spend per person if you can't hit 150 guests. For example, if they want you to spend a minimum of 150 portions that cost $100, ask if you can have a 90 person party at $167 per person instead by upgrading their meal. This could be an extra course, a premium vs standard bar, more late night food, etc.
You can certainly try to negotiate but like I said, it's peak wedding season during your time and if the venue is one that's in demand, it's very unlikely you can get out of their minimum as the demand means they can command it since people are willing to pay for it.