I've made a lot of alts and that never has been a problem, just post some vaguely affirmative comment in popular thread and boom, 50 karma.

The whole idea is utterly stupid because it is far too easy to get on popular low effort subreddit and get some points for posting absolute garbage, then go and bother people in niche subreddit.

While karma requirements are easy to overcome, many spammers don't go through the effort to do so.

They take about 2-3 days of significant engagement for most subreddits karma requirement. Some take weeks of existing.

If you have to wait 2-3 days to comment the moment as they say has passed and very few would read it.