After the 1974 revolution that overthrew Fascism Portugal had a couple of decades of being quite Leftwing during which it massivelly invested in Education, including tertiary education, so the generations now in their 50s and younger almost universally learned English language at a school and include a large proportion of people with higher degrees.

There was also a period in the 80s/90s with some investment in adult education and that coincided with the fast growth of personal computing around here.

Then add to that that fixed Internet is quite cheap (a TV + unlimited 200Mb fibre + fixed phone line monthly packets costs around €45 and recently some even cheaper and faster providers have entered the local market, so for example I pay €15/month for 1 Gbps fibre) and almost universally fibre, possibly because unlike in countries like the UK the incumbent telecom operator in Portugal for whatever reason didn't really manage to or tried to block the replacement of older internet access technologies with Fiber (I was living abroad at the time, so I don't know for sure, but the period with maybe the only half-way decent non-crooked government - led by the guy who now heads the UN - kinda overlaps with the time with fixed Internet started spreading, so maybe they actually did some proper, Northern-Europe quality legislation around data access infrastructure which yielded a far more competitive market than you see in places like the UK).

I suspect some or all of these things explain why in this specific domain Portugal looks a lot more like smaller Northern European countries than it's tradition of being a Chunk Of South-America in Europe would lead one to expect.

That is actually a very compelling explanation. Although I guess the data it is based on would be a bit less relevant considering they are from 19 years ago...