No. As far as I know, they haven't said anything about this. Neither did OpenAI about gpt-4-32k.
MosaicML did say something about MPT-7B-StoryWriter-65k+: https://www.mosaicml.com/blog/mpt-7b. They are using ALiBi (Attention with Linear Biases): https://arxiv.org/abs/2108.12409.
I think OpenAI and Anthropic are using ALiBi or their own proprietary advances. Both seem possible.