So in my case I am the only user on my instance so I am certainly not going to be hammering a bunch of instances just to send me updates of whatever total number of communities I'm subscribed to.

You don't understand how federated replication works. It doesn't occur on-demand when you read a post, it occurs when the instance hosting the community gets a post, comment, or vote. The federation load you place on other servers has nothing to do with how many users are on your instance or how much they read... it has everything to do with how many communities they subscribe to. This script is literally signing you up to proactively receive the firehose of every post and comment in the lemmyverse, without regard for what you actually look at.

I completely understand the idea of the app, and your confusion about how much load it generates is exactly why it's such an irresponsible idea. If you want to fill the timeline of your small instance, do so by subscribing to specific communities you're interested in until your timeline becomes active enough for you. Subscribing to 100 communities you care about will result in a very lively feed of stuff that is interesting to you, while generating a tiny percentage of the federation load this approach does. Carpet bombing the entire lemmyverse with subscriptions you cannot read is madness. It's like writing a reddit app that downloads everything ever posted to reddit to your phone to save you the trouble of picking subreddits to follow. It's bad for reddit, bad for your phone, bad for your isp, and a bad idea all around. If I were running a large instance, I'd defederate with any tiny instance I observed subscribing indiscriminately via this script. It's abuse.