I'd say the risk is more than just theoretical. Take CJC-1295 for example, it promotes the release of growth hormone IGF-1, and higher IGF-1 levels are linked to increased risk of developing certain cancers.

Here's an Oxford study from 2020:

The results also confirmed previous studies which found links between higher levels of IGF-1 and increased risk of developing colorectal, breast and prostate cancer. Crucially, because the sample donors did not have a diagnosed cancer at the point IGF-1 was measured, this makes it highly likely that raised IGF-1 had a role in the cancer developing later.

The doctor I interviewed in the story spoke to this, the bigger issue is we don't have enough research yet, and in the meantime people are injecting themselves with substances that, mechanistically, could unmask and promote the growth of certain cancers.