Yes, that's certainly possible, if their production lines are all fully occupied producing Sony mount versions. But even if that was the case, it would make sound business sense for Sigma and Tamron to "get in as early as possible" on a new opportunity, and at the very least to announce that RF and Z mount lenses were in the pipeline.
I'm sure there must be "lines of communication" open between Canon, Tamron and Sigma whereby the subject has been discussed, so either some kind of "understanding" has been reached, or Sigma and Tamron are fearful of compatibility issues with future bodies. The whole thing seems very hush-hush, so I'd guess there's a lot happening behind closed doors - and an agreement not to disclose publicly.