Ye, especially with something like Netlify or Vercel, I'd bet the vast majority of the costs are CDN bandwidth, and it never even touches the functions themselves. Could just as easily be in this situation with AWS CloudFront and an S3 bucket.
I have seen AWS Lambda generate large bills even with the 1000 concurrent executions limit, but with the help of upstream services. My memory of it is hazy, but I think over a weekend, a function with some dodgy retry logic managed to generate something like 21TB of CloudWatch logs.