It seems that it would otherwise silently skip the rescan for pre-backup used keys that are still waiting to be generated into the keypool?

See also #11249 
{
    if (data.exists("timestamp")) {
        const UniValue& timestamp = data["timestamp"];
        if (timestamp.isNum()) {
            return timestamp.getInt<int64_t>();
        } else if (timestamp.isStr() && timestamp.get_str() == "now") {
            return now;
        }
        throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Expected number or \"now\" timestamp value for key. got type %s", uvTypeName(timestamp.type())));
    }
    throw JSONRPCError(RPC_TYPE_ERROR, "Missing required timestamp field for key");
}
RPCHelpMan importmulti()
{
    return RPCHelpMan{"importmulti",
                "\nImport addresses/scripts (with private or public keys, redeem script (P2SH)), optionally rescanning the blockchain from the earliest creation time of the imported scripts. Requires a new wallet backup.\n"