Otdfbt

Am I missing something but is there no way to add a route via CloudFormation to the default route table that comes provisioned with a VPC?

Nah you can't, there's nothing to refer to anyway (e.g. logical ID). Just create your own main table ;-).

This is probably one of the reason it can't be used:

One way to protect your VPC is to leave the main route table in its original default state (with only the local route), and
explicitly associate each new subnet you create with one of the custom route tables you've created. This ensures that you must
explicitly control how each subnet's outbound traffic is routed.

You can define each component by yourself in case you need to implement that setup via CloudFormation. Just create your own VPC, Internet Gateway, Subnet and Route Table. Then you need to explicitly declare RouteTableAssociation for the specific subnet and create a public route for that table. Here's an example

AWSTemplateFormatVersion: '2010-09-09'
Description: Example
Resources:
 myInternetGateway:
 Type: AWS::EC2::InternetGateway
 Properties:
 Tags:
 - Key: "Name"
 Value: "a_gateway"

 myVPC:
 Type: AWS::EC2::VPC
 Properties:
 CidrBlock: 10.0.0.0/24
 EnableDnsSupport: true
 EnableDnsHostnames: true
 InstanceTenancy: default

 # Attach Internet gateway to created VPC
 AttachGateway:
 Type: AWS::EC2::VPCGatewayAttachment
 Properties:
 VpcId:
 Ref: myVPC
 InternetGatewayId:
 Ref: myInternetGateway

 # Create public routes table for VPC
 myPublicRouteTable:
 Type: AWS::EC2::RouteTable
 Properties:
 VpcId: !Ref myVPC
 Tags:
 - Key: "Name"
 Value: "public_routes"

 # Create a route for the table which will forward the traffic
 # from the gateway
 myDefaultPublicRoute:
 Type: AWS::EC2::Route
 DependsOn: AttachGateway
 Properties:
 RouteTableId: !Ref myPublicRouteTable
 DestinationCidrBlock: 0.0.0.0/0
 GatewayId: !Ref myInternetGateway

 # Subnet within VPC which will use route table (with default route)
 # from Internet gateway
 mySubnet:
 Type: AWS::EC2::Subnet
 Properties:
 AvailabilityZone: ""
 CidrBlock: 10.0.0.0/25
 MapPublicIpOnLaunch: true
 VpcId:
 Ref: myVPC

 # Associate route table (which contains default route) to newly created subnet
 myPublicRouteTableAssociation:
 Type: AWS::EC2::SubnetRouteTableAssociation
 Properties:
 RouteTableId: !Ref myPublicRouteTable
 SubnetId: !Ref mySubnet

This way you'll be able to use created route table (in the example above it's used to forward traffic from Internet Gateway)