I generally treat claims such as Suleman’s with considerable skepticism. In years of watching people math out their returns on L4 runs and burner missions, it’s rare to see someone credibly claim more than 150M/ISK per hour on L4’s and about 200M/hr on burners. Although people sometimes pull out a couple ideal hours where they got the best of everything and claim 200ish on L4s and 250-300 on burners.
The credible claims are also from people who have really maxed out everything and every resource to pull those numbers, and they rarely do so long term because it becomes too grindy.
People with longer term routines that involve lots of mission running, in my experience, are more likely to claim 80-100M ISK/hr on L4s if they are cherry picking, and about 60M ISK per hour if they are running a more normal routine. I haven’t followed burner runners as much but 100-120M per hour seems a fairly frequent claim. (These numbers are still by people who are extremely efficient and highly skilled, not ‘casual’ mission runners.)
Given that, the math on 1B ISK per day mostly from missions seems like somebody’s fudging their numbers a fair bit.
For the OP, a not-too-outdated discussion on solo mission running might be useful:
L4 Missions 100M Isk per hour Solo