Stack + reach for beginner bike

Stack and reach isn't about what you are used to but is all about the actual bike dimensions. So yes they matter to a beginner. Now some could probably argue they matter more to a beginner for the exact reason that the athlete doesn't know how the bike should feel so they can't make tweeks to "wing it" by doing something like changing stem length.

YES. It matters.  Here's a quick primer / DIY test.

Get fit now, ride happy for a long time.

Don't get fit now, spend many months posting on here / ST asking "what's wrong with my position?" and end up getting the fit (and hopefully not a whole new bike because the one you bought doesn't fit) anyways.

Choose carefully.

And if you can't get fit because you are buying a bike online because it is about half the price? But you can get fit after you get the bike? Then what?

Then you roll the dice and take your chances. If it fits you got a great deal. If it doesn't fit, then you have to sell it and get another one which brings you up to the price you could have paid originally, except had you bought it from your LBS originally they would throw in yearly checkouts, occasional tightening of stuff and the occasional really good deal. Plus having someone to take it back to if you were unhappy with it.

But it's your money. Enjoy.

The idea is to get fit before you buy so you can use your can compare your fit coordinates against published frame geometries.  Adjusting aerobars, seat heights etc. is only part of the equation.

Listen to those wise words!!!

This is exactly how everyone should buy their first bike... understand what will work for you and THEN go get it... rather than trying to buy something and then making it work (which will pretty much always end up in more compromises)