If Shopify "hijacked" it by making a better product it would be more okay. When they however outright just ban competition, that's anti-consumer behavior and should be stopped.
No, in the part "April 2020 - Shopify hijacks Upsell X" they are requesting him to "not distribute mass solutions via private apps to Shopify merchants in order to bypass checkout". Shopify had their own checkout from the beginning and the whole purpose of his application was to bypass the existing solution. I assume there were Bulgarian payment providers connected to Shopify in 2017, but according to himself these payment providers wouldn't accept their business idea to run a pharmacy as an e-commerce. Therefore, he made a workaround.