Why would they refuse to sell you Directv if you are at an address with AT&T TV? You can't get NFLST with AT&T TV can you? You can't get 4K...
They will never be exactly equivalent (where's the support for the LCC with AT&T TV, does the cloud DVR support everything the real DVR can do, etc.) so they can't consider them equivalent.
All they have to do is say "since you are at an address with AT&T fiber we recommend AT&T TV" and if you insist you want Directv say "OK but since you are at an address with AT&T fiber you will have to pay for the install if you want Directv". Maybe those people will be the first to have to pay for install, though I expect that will become the norm for Directv within a couple years. Then they don't have to care what people choose, because they won't be on the hook for the up front cost.
They might refuse to sell DirecTV at addresses eligible for AT&T TV because they plan to hold onto the AT&T TV service indefinitely but they may choose to somehow spin-off or sell DirecTV in the next few years. In which case, they'd want to shift as large a percentage of their cable subscribers as possible over to AT&T TV.
As far as the examples you give of AT&T TV not being equivalent with DirecTV, there's no reason to think any of those are permanent. We know that the AT&T TV box supports 4K HDR and the CEO has said those formats will be supported eventually on the service, so I expect that's just a matter of time. Might even be announced when AT&T TV launches nationwide in Feb.
As for NFLST, who knows if that will even be available on DTV come this fall and, if it is, if it will be again in 2021. (And if the deal does get renewed with AT&T for 2021, I'd definitely bet it'll be available on AT&T TV at that point. We know, after all, that the NFL is eager to get the package on some kind of streaming platform.)
As for an LCC, for all we know AT&T has a network-based tuner in the works, something similar to Sling's AirTV 2, that could work with AT&T TV. And the vast majority of potential subscribers to either DTV or AT&T TV don't know what an LCC is or even think about trying to integrate OTA TV in with cable, so it's not a big deal either way.
AT&T TV's cloud DVR is a trade-off versus the Genie; it has more storage, can record an unlimited number of simultaneous channels, and is accessible anywhere over any internet connection, but the cloud recordings auto-delete after 90 days. (Given that Comcast's cloud DVR has a 12-month limit and YouTube TV has a 9-month limit, I really think AT&T TV is hurting itself there.)
It's possible that DTV will move to a policy where some or all new customers have to pay for installation costs themselves up-front but I really doubt it given that DISH will still be touting free installs with a 2-year contract. I think those installation costs will continue to be spread out and baked into the average monthly price of service paid over the 2-year contract period, which is why the monthly cost will be higher for DTV than AT&T TV. And then after the 2-year contract is up, DTV customers who call in and complain about the price and ask for a discount will be told to switch to the cheaper AT&T TV if they live at an eligible address.