It appears that even the conservative timeline was too fast for Tesla as Musk has now clarified the issue, explaining, “Demand is no problem, but near-term cell supply makes it hard to scale Semi. This limitation will be less onerous next year.”
Part of the reason for the conservative (for Tesla) timeline for the Semi was always the lack of battery production capability. Each Semi will need at least six times as many battery cells as the Model Y, which is understandable. That comment from Musk was regarding a report that Tesla had just received an order for ten trucks and two chargers from a California-based company.