The US companies talking about “pacing” aren't actually talking about slowing down their own development efforts, they are talking about slowing down competition. That is, specifically, they are seeking to have government, on the pretext of “safety”:

(1) Give the big AI incumbents an anti-trust exemption so the they can coordinate without it being an illegal agreement not to compete, and

(2) Adopt mandatory supervision (by giving priveleged access to monitors) to the shared safety protocols of the big labs, by a nonprofit funded by the big labs, of everyone training, distributing, or hosting models.

(3) Adopt a policy of seeking international agreements to extend substantially the same rules to foreign actors training, distributing, and hosting models.

The part they are doing voluntarily isn't to promote the lobbying effort for these mandates isn't slowing down, it is giving their pet nonprofit the access for “independwht supervision” of their own operations to their own existing safety rules.