Apple - Is Apple using the updated 2018 MacBook Pro keyboard when replacing older keyboards under the new keyboard service program?

Nope, sadly not.

From here:

Apple has been servicing affected keyboards free of charge, with the process involving the replacement of one or more keys, or the whole keyboard. For the MacBook Pro, the replacements are second-generation keyboards -- often the 2017 variant with slightly different markings on the Control and Option keys.

When asked if Apple Stores and Apple Authorized Service Providers will be permitted to replace second-generation keyboards on 2016 and 2017 MacBook Pro models with the new third-generation keyboards, if necessary, Apple said, no, the third-generation keyboards are exclusive to the 2018 MacBook Pro.