In which Issue? At Issue 1, Tankers did pretty low damage. I recall at least two damage boosts for them, and one was pretty significant. After that, yeah, Tanks became a much better soloing AT.
In a sense, but compared to what? At release, the tanker damage mod was 0.65, which is 81% of the value they were buffed to at 0.8. But Scrappers were also buffed, after Tankers, from 1.0 to 1.125. They also got pervasive crits in I1. Although a few powers already did crits, those were also buffed. Even assuming a 5% crit rate (it was 10% vs higher ranked targets) the effective Scrapper mod was closer to 1.18. So looking at the modifiers as a base for comparison, tanker damage was about 65% of scrappers at release, and 68% of scrappers after the first round of damage buffs on both. Relative to what everyone else was dealing out, Tanker damage wasn't actually significantly lower at release than it was later on.
What's more, although they did do low damage at Issue 1, interestingly their damage was more effective at release prior to Issue 1 for a not well-known technical reason. The purple patch wasn't introduced until after release and just before Issue 1. Prior to the introduction of the purple patch, there was no serious impediment to attacking targets large numbers of combat levels higher than you were, provided only that you could survive them. Tankers with their high defenses and health could far more effectively attack targets as high as +8 or even +10 and gain huge rewards for it. Even with their lower damage, they could in many cases gain higher levels of rewards by leveraging their high defenses and using their lower offense against juicier targets. It wasn't until the purple patch was added to the game that Tankers could no longer translate their superior defenses into ultrahigh reward rates irrespective of offensive output.
Completely separate from that, Tankers were still strong solo archetypes compared to most of the others because of another historical issue: prior to the devs adding difficulty sliders, all missions including solo missions always spawned bosses (unless they were explicitly coded to *not* spawn bosses). That made soloing significantly more difficult for the average player playing archetypes like blasters and defenders. Tanker damage made soloing slower, but datamining proved that defenders to a smaller degree, and blasters to a ridiculous degree, were dying at far higher rates than other archetypes in solo missions.
For a significant time after release, I would say that as an archetype as a whole, Tankers were the second best archetype to solo behind Scrappers. Blasters were far too fragile. Most defenders had similarly low damage and were more vulnerable. Most controllers also had low damage prior to pets (and not all controllers had pets) and only some combinations had effective means of boosting offense in solo missions.
My pre-I5 soloing chart would be:
Scrappers
High level Controllers
Tankers
Defenders
Low level Controllers
Blasters
Of course, specific builds and specific players could individually succeed with anything. But averaged across the entire playerbase, I'm pretty confident that's how effective the original archetypes were at soloing from I1 until at least I5.