Afaik, the point of cardio hurting "gains" comes from three points.
1- You need a lot of calories in order to gain muscle mass, and doing cardio burns calories. Therefore, it is counterproductive to your goals. This is relevant only for bodybuilders though.
2- Recovery is best looked at as some set amount of resource you have. Anything physical you do will cut into it in some form, though obviously it varies quite a bit depending on the activity you do.
If you're looking primarily for strength and your ability to run 2-3+ miles is not something you care about at all, then why spend recovery resources on that when you don't have to?
And since you can lose fat and build a decent heart just from the act of walking 3-4 miles fast on a daily basis, while not using a whole lot of your recovery resource, why not do that instead of jogging or running?
3- Running, or most activities where you just work at a pace for many hours straight, is said to lead you body into a catabolic state. Which is obviously bad for both muscle and strength improvements.
But know that virtually every article (at least the ones I've read) that rant on the negatives of long cardio is that they're nothing more than rants.
They don't actually specify anything and just go on and on about how "cardio", which they never bother to define anymore specifically than talk about marathoners, kills strength and muscle growth through what I wrote above. And then they talk about the virtues of walking and HiiT work and how everyone should do those instead.
But the fact is the "cardio" you do matters. Is it a "short" session many hours after you lifted, where you jog 3-4 miles at a lively pace? Or it is 10+ miles? Jogging 3-4 miles won't hurt your gains, provided that you account for it in your eating. The improvement in your heart and lung capacity will probably help your training.
Interval training, particular HiiT, can fuck you up far far more than random-ass jogging. Because HiiT is comparable to what weight-lifting would be doing. If you program it wrong, then it'll burn you hard and you won't recover enough.
