Will any ScienceCloud multitenant customer (or non-Hub on-premise customer) that existed prior to 2023 SP2 get the Floor NodeType in their system when they are upgraded to 2023 SP2 or later?

No - the script to add the Floor NodeType in 2023 SP2 only applies to baseline schema (i.e. any schema marked as "isNew" in ConfigUX), so existing customers with the proper configuration (i.e. not marked as "isNew") should not be affected.