Are these legit Google reviews for a pool service?

Probably not. I don't trust online reviews, too many companies out there that can be hired to post fake 5 star reviews.
No way to tell. Reviews could have been submitted by actual customers who were encouraged to give a 5 star review in exchange for a discount on their next pool cleaning bill.
Online reviews are complete horse excrement these days. Its so easy to bribe the customer with discount/free stuff to get reviews, or just hire it out. The honest businesses that dont do it only get negative experiences posted. People rarely just take time to review things unless it was a mind blowing experience. However, they jump at the chance to post negative things.
Fake or not, I’d give the place a try. Perhaps schedule them to come out while you’re home and go from there.

New business starting out. I see no issue with the fake reviews. They have to make themselves look good somehow.

On a side note, chlorine has doubled in price and there’s purchase limits in stores. Get it while you can. The wife and I were considering a small cheap above ground pool. I don’t see it happening.