I think it's to reduce expectations since so much is out of his control. I mean, remember last year? Everyone hyped it as a WR attempt, he did too, and then it fell short, all very much due to circumstances out of his control. It's almost, in a sense, an attempt not to jinx it.
I would agree if the margin between his PB and the WR was a minute or maybe even 30 seconds. But it’s 8 seconds!
If you say PB instead of WR, you make yourself sound like a fool. If you want to lower expectations just say:” I will try to go for a WR, but many things are out of my hands as we’ve already seen in previous marathon. This would sound like true modesty. But the way he says it- we aren’t idiots to buy a claim like that. It’s fake cheap modesty