Sinema is very obviously not going to get votes from either partisan Democrats or MAGA Republicans. The people who are going to vote for her in a race with Gallego and Lake are Biden-McSally voters. And given a binary choice between Gallego and Lake, I think most of those people would vote for Gallego.
I just don't really see an argument for the idea that Sinema running as an independent would result in her taking more votes from the Republican candidate.
Sinema has a higher favorability rating among Republicans than she does with Democrats