SP's overtaxed SA production capabilities are definitely an issue.
However, back in March, an Aug/Sep slot was perfectly reasonable given what was already in their queue. But MassDrop simply chose not to slot it there, for reasons only known to them. The longer they waited to slot it in, the longer the queue grew ahead of it; in a sense, it is an outcome born pretty much out of their own choices, not SP's. Apart from Nantucket Selectric, I don't think SP is making SA sets for anyone but MassDrop. So nearly all of SP's production backlog comes from MassDrop. They are steering the SA ship at this point, not SP.