The INSPIRE thematic WMS layers (admin units, hydrography, geographical names, buildings, cadastral parcels) are served by Maanmittauslaitos in ETRS89 (EPSG:4258). The wrapper translates WGS 84 bounding boxes to the EPSG:4258 axis order automatically; the two datums are practically identical in Europe. See FAQ: EPSG:4258 vs EPSG:4326.
The upstream Maanmittauslaitos WMTS service requires a Referer header; a server-side proxy was previously available at /wmts/fi/… but WMTS tile proxying has been removed in this version.