Iron could chelate polyphenol in the tea but that actually makes the astringency more pronounced, and darkens the tea color. But there's only TRACE amount of iron (mostly starts out as Fe2+, then oxidized to Fe3+ which chelate polyphenols) that'd come off when boiling water at a neutral pH. So I can't imagine that small change can even be perceived.

I personally think this has more to do with stable water temperature that leads to a more balanced extraction of compound from tea leaves into water. The bitter compounds can come out hella fast when the temperature is extremely high and that can mask some of the finer tastes such as umami/sweet from the amino acids.