Is it unusual for populations of archaea to be found living inside trees?
The exact term — wetwood, is used only once in the original article (Yip et al., 2018).

Others have found that water content of heartwood tissue was the best
predictor of methane flux across multiple plant species (Machacova et
al., 2016; Wang et al., 2017), however this was not a strong predictor
of methanogen relative abundance in our analyses. It is possible that
within trees characterized by wetwood such as Populus deltoides,
other factors may become more important than water content above a
certain moisture threshold.

However, if you look at these lines from the introduction of the paper, it seems that wetwood just means wood that has a high water content. It definitely does not refer to a disease or infection.

Indeed, both mechanisms may be important depending on tree species
(e.g. those containing dry and dense vs wet and porous wood) and
ecosystem under study (e.g. wetland vs upland forest soils).



Based upon past work and our own informal observations of odors
associated with tree coring at these sites indicative of fermentative
environments, we hypothesized that an anaerobic community including
methanogenic archaea would dominate woody tissue, particularly in
heartwood tissue which is often water‐soaked.


Now coming to your question on whether it is unusual for populations of archaea to be found living inside trees.
Roots have been known to harbour microbes; plants establish this symbiotic relationship with the microbes for nitrogen fixation (See mycorrhiza and root nodules). So, it may not be very unusual that archaea interact with plants but it may possibly be unusual that they are living inside this part of the tree. Yip et al. (2018) highlight that although microbial composition of the wood has been studied in the past, these studies have been done ex situ. This means they are the first to study these microbes in situ.

Microbiological investigations of living wood have been conducted in
the past (Zeikus &amp; Ward, 1974; Zeikus &amp; Henning, 1975; Ward &amp; Zeikus,
1980; Schink et al., 1981a,b, 1982), but these studies primarily
focused on ex situ wood incubations and the isolation of microbial
representatives, rather than overall community characterization across
tree conditions. For example, Zeikus &amp; Ward (1974) demonstrated that
methane production occurred from extracted wood tissue from Populus
deltoides and were able to isolate pure cultures of a new species,
Methanobacterium arborphilicum (Zeikus &amp; Henning, 1975), later
reclassified as Methanobrevibacter arboriphilus (Balch et al., 1979).

So, it seems that people have been working on this area since 1970s.
However, it also appears that there is not much research on archaea as a part of the plant microbiome (Taffner et al., 2018).

IMPORTANCE: Archaea are still an underdetected and little-studied part
of the plant microbiome. We provide first and novel insights into
Archaea as a functional component of the plant microbiome obtained by
metagenomic analyses. Archaea were found to have the potential to
interact with plants by (i) plant growth promotion through auxin
biosynthesis, (ii) nutrient supply, and (iii) protection against
abiotic stress.

So whether this is unusual or not depends on whom you are asking (hence a bit opinion based).