The PHB beastmaster lets you make an attack at the same time as your pet if you have the Extra Attack feature. Now, its true that this technically not an attack action, and will have some odd interactions with some other rules, but realistically? Its little different than the ranger giving up one of their attacks and giving it to the pet.The current Ranger technically gives up an action to make the pet attack, not an attack. It’s a subtle difference that would have allowed the two to attack side by side at 5th level (or at 3rd if it worked with TWFing). It doesn’t work with the Warlock because EB doesn’t work like Extra Attack.
I suppose if you want to be technical, since its not an attack action, you can't TWF or use Swift Quiver and other things to make a bonus action attack either, but that's where I'd just make the ruling that you can, because otherwise it's counter-intuitive to me.
Even in this case, however there were issues. People didn't like giving up their attacks to deal less damage through a pet. Or one that could die so easily. The inability to apply Hunter's Mark damage hurt as well. The list goes on, but I think it effectively came down to the feeling that the pet was diminishing the Ranger's abilities rather than complementing them, and the ranger would be more effective in both exploration and combat without the pet.