I've been keeping my eyes open for new rubs for the last 6 weeks while in the woods - most of my current locates are on conifers, which surprised me a bit.
Those are really big rubs though - I'd check for hair too - that will confirm the animal. The trees appear to be alder - which in the past I thought the deer preferred - but I was surprised to find that elk hit the big alder really hard too, only the rubs tend to be higher up and often 24 inches or more, top to bottom. Sometimes a deer will rub just below or at the bottom of an elk rub too, which makes it really confusing.
The height of the rubs appear to be appropriate for blacktail - a trail cam next fall in addition to a tree stand sounds like a winner to me. I think you're probably right - it's likely a massive old buck!