Note that, by the time this answer was finished, LangLangC's adding of context made it clear that Mr. Steyn was arguing Mr. Mann's methods specifically (in context of a slander lawsuit levied by Mr. Mann), not the conclusions drawn or climate change in general, in this particular context (although it is pretty clear that he does so in the rest of his book).
As for the issues of Mr. Steyn and Mr. Mann about that one tree, I'd refer to the findings of the lawsuit or argue to "close for being about unresolved current events".
As for the tag attached to the question, "climate change", I find the issue to be...
Irrelevant for a discussion on climate change.
Recognised smear campaign.
This is part of a smear campaign "by fossil fuel industry–funded lobbying groups attempting to cast doubt on climate science" (1) by proxy of a 20-year-old paper.
Check the relevant Wikipedia articles on Hockey stick graph and Hockey stick controversy for an extended and well-sourced discussion of the matter.
Data scarcity documented in paper, data points not essential or even especially helpful to the conclusions drawn.
This is figure 5 from the 1998 paper that is being referred to: (2)
It is evident that "the entire pre-1421 graph" is about 3.5% of the total graph at the beginning, and that it is actually showing a warmer period (i.e. not cherry-picking a section that would be strengthening the point being made).
The text on the same page reads:
The reconstructions discussed here are derived using all indicators available, and using the optimal eigenvector subsets determined in the calibration experiments described above (11 from 1780–1980, 9 from 1760–1779, 8 from 1750–1759, 5 from 1700–1749, 4 from 1600–1699, 2 from 1450–1599, 1 from 1400–1449).
And, a bit earlier:
(6) The multiproxy network of 22 indicators available back to 1400 resolves only the first eigenvector, associated with 40–50% of resolved variance in NH in calibration and verification. There is no useful resolution of spatial patterns of variability this far back. The sparser networks available before 1400 show little evidence of skill in reconstructing even the first eigenvector, terminating useful reconstruction at the initial year AD 1400.
I.e., the paper is quite candid about the deteriorating amount and quality of data available at the time the further back you go. (Which makes it "funny" that another angle of criticism had been that the graph were cherry-picking by not going even further back in time to show the Medieval Warm Period.)
The paper also makes no further explicit reference to the 1400-year-mark or early part of the graph, i.e. its findings are in no way or form "based" on the inclusion of that (valid) data point. Assuming this to be "publishing rubbish", as the OP detailed in one comment, is a far cry from what the paper actually did.
Consistent with later findings.
Since that paper was published, more than two dozen other large-scale temperature reconstructions have been conducted.
The curve shown in graphs of these reconstructions is widely known as the hockey stick graph because of the sharp increase in temperatures during the last century. As of 2010 this broad pattern was supported by more than two dozen reconstructions, using various statistical methods and combinations of proxy records, with variations in how flat the pre-20th-century "shaft" appears. (3)
The temperature record of the past 2000 years, and especially its eponymous hockey-stick form, has wide scientific (and popular) consensus.