This is fuzzy logic.

20x 1 SPS boost = 20x SPS boost?

No.. that's not how it works. Applying a 1x multi. to a $400 account or 1x multi. to 20 $20 accounts is effectively the same. The multipliers don't add up

Because if I would play with a real CL silver deck in silver, I would earn much less than him with his 20 accounts.

What are you trying to compare? A bot in wild vs human in modern? I just checked this account and it earned 25 SPS in the past month, and 15 SB cards. I don't know what the ROI is but it's not amazing. In fact, if you look at SPS earnings alone, this account hasn't earned back the $10 spellbook cost after 2 YEARS of running. The perception that bot farms have amazing ROI is just false.

he can simply bypass the SPS and even ends up with more sps boost

Not true, as explained above. If this was the case, it would be a major design flaw