Deplatforming and demonetizing reduce their reach. Demonstrably so. The group may get hardened views as a result, but they are much smaller as a result too. Conversely not doing this and instead doing what Substack is doing gives them a much larger audience for recruitment and influence. You're trading off deradicalization for their recruitment.
And as Derek Black's story shows deradicalization can also come via this stigma. His journey away from the KKK came from the ostracism and condemnation he experienced at university.
This reminds me of a saying...it goes something like "if you find yourself sitting at a table with 9 nazis, there are 10 nazis at the table"