I think Tony's being a little too simplistic in his thinking really:
1. There is clearly far less room for parties to run up such huge leads if there are only really two parties (south of the border) with any meaningful support. If the Tories can rely on, say, at least 40% support at the moment, then a 15-point lead is only possible if literally nobody is an undecided voter.
2. The Tories probably *can* be fairly safe with that level of support at the moment. Labour and the Tories are diverging in their policies, and there is no centrist party of note, so there's nowhere for wavering Tories to go, really.
3. Also Brexit.
4. Presumably Blair's point was to criticise Corbyn, but it seems odd timing after what was a relatively successful result for him (compared to the widely-predicted "Labour will be steamrollered and lucky to get to 200 seats" benchmark). I'm not sure what Corbyn can do to attract his opponents, some of whom are now so ideologically opposed to Corbyn and what it stands for that they are drawing comparisons with Stalinist regimes etc. Whether or not such criticisms are fair, they're rather difficult to shake. I suspect that many anti-Corbyn supporters wouldn't vote Labour even if held at gunpoint.
And vice versa, actually. Politics has become very partisan, and the parties are reacting to this. Brexit showed that there were deep divisions in the UK in political vision, but also (irritatingly) that these divides were pretty much 50/50, so that neither side can expect to have massive leads over the other.
In short, then, I think Tony Blair's expectations of a 15+ point lead refer to a time in politics when this was actually possible, and for the moment -- and for many more reasons than Corbyn -- such an expectation seems unreasonable.