I can't even begin to imagine what those 100k people actually do. For starters, none of the telcos actually develop their own equipment - they buy pre-made from vendors like Ericsson. Often that includes ongoing maintenance too. The only "engineering" is building the back-office and customer-facing UIs, and even that is often outsourced (as a rule of thumb, if something can be outsourced, telcos will do it: https://berthub.eu/articles/posts/5g-elephant-in-the-room/).
Customer service might be part of that number (assuming that too isn't outsourced), but even then 100k feels extreme.
10k is ok although leaning on the more bloated side. But 100k?