When people are blocked for using a VPN it's usually because that IP was used in an attack at some point and added to a blacklist and since no one really owns it its never been contested, or its been used in multiple attacks and considered permanently added. Since a VPN provider's entire purpose is to hide what you're doing it's difficult for a provider to keep its users from abusing that IP.

So while it's possible to get a list of IP's that are owned by VPN providers and proactively block them it's generally only intended to block IP's known to be abusive.

Lemmy instances are just blocking IP's used in abuse, Reddit is actively trying to prevent robotic scrapers to keep their data more valuable to sell to AI companies so they are only interested in blocking VPN's they suspect are trying to scrape data and not a logged in user who happens to be using a VPN because if they know the user and are using a VPN and start scraping they can just ban the user.

Tl;Dr its about intent; Lemmy is preventing abuse vs. Reddit is protecting the value of its data for sales.