Actual analyses done on this topic by Pew, Breugel, and National Affairs suggest this effect is largely not true. When considering the entire electorate, a significant number of non-voters lean Republican or are politically unaffiliated and would not support the democratic party.

Further research indicates that, despite popular belief, higher voter turnout does not consistently benefit either party across the board. Over the past 70 years, there has been no strong correlation between increases in turnout and the Democratic vote share in presidential or midterm elections.

I am not sure how to find this. I actually already looked at a Pew article earliee today and didn't happen to see it there.

https://www.pewresearch.org/politics/2023/07/12/voter-turnout-2018-2022/

I'll link it again and look through it again to see if it's there.

Further, your second point is lacking any source including an author.

Last, whatever source you're quoting specifically important because I'm highly curious what the verbiage actually is. There is a world of difference between someone who "leans" Republican versus someone who will never vote Democrat.

Eta, from above Pew source:

Adults who voted in at least one election during the period divide evenly between Democrats and independents who lean toward the Democratic Party or Republicans and Republican-leaning independents in their current party affiliation (48% each). The subset who voted in all three elections are similarly divided (49% Democrats, 50% Republicans). Citizens who did not vote in any of the three tilt Republican by 46% to 41%.

Reflecting these patterns, Republicans won a majority of votes among those who said they voted in person on Election Day, 60% to 38%. Democrats won – by an identical margin – voters who said they voted by mail or absentee ballot. Those who said they voted in person before Election Day were divided: 53% supported Republican candidates, while 46% voted for Democratic candidates.

Perhaps if Dems were able to pass easier mail in voting or better legislation generally, people would be more likely to vote for them