Product manufacturing isn't like purchasing consumer goods. A factory isn't going to constantly crank out parts for a product that is only built in limited runs. Factories often tool up to manufacture limited runs of a product (or product component), then when the run is concluded the factories tool up to manufacture a different product (or product component). Depending on how manufacturing agreements go, a factory could be booked months or even years in advance regarding the manufacture of various product runs.
So we could very well have a scenario where Sennheiser's preferred manufacturer won't have an opening in their schedule until October, for example. Once the factory has fulfilled all their prior commitments, they can then tool up to make Sennheiser's parts. The factory will be able to make as many parts as Sennheiser needs within a matter of a few weeks, but the catch is that Sennheiser needs to commit to a production run by a certain deadline, so that the factory can schedule the production run and also provide an estimated cost for the production run. Once the deadline passes, the factory is committed to manufacturing the number of units specified by Sennheiser and nothing more. If Sennheiser tries to change the production run after the deadline, that complicates the supply chain, which could increase the cost, as well as force the factory to delay the production runs of their other customers, which won't go over well.
If it takes one week to make 10,000 parts and two weeks to make 20,000 parts, but there's a three month delay before the production run can even start, that's your reason for the long lead time. It's not the short amount of time needed to actually make the parts, but the long wait for Sennheiser's turn in line. For an alternate example, think of going to the dentist. It might only take him an hour to work on one of your teeth, but you're waiting three weeks to see him because he has a busy schedule. If you call him in advance and tell him you have 5 teeth to work on, he can do that and it will take no more than five hours to do, but you still have to wait the three weeks to see him. And you can't wait until the last minute to tell him about the extra teeth, because if he's only allocated one hour to work on one of your teeth, you can't expect him to delay or cancel all the appointments after you. You need to let him know to budget that extra time before his schedule gets filled, otherwise you'll have to make a separate appointment for the other teeth, which means more waiting.
So to sum it up, it's not unusual in manufacturing to be able to have parts manufactured quickly, but have to wait a long time before the parts start getting manufactured. I see no evidence that Massdrop and/or Sennheiser are lying. They're just trying to put together the biggest production run they can before they have to commit to manufacturing, because once they commit, it's too late to change anything.