There's a persistent myth that "scouting" is all about uncovering a player whom no one has ever spotted before. That's not, realistically, what football recruitment is about at this level and in this century. Sure, it applies to the some of the 16-17-year-olds whom we snap up and park in the academy - but not even all of them.
So the fact that Alderweireld played for Atletico Madrid and Ajax isn't a measure of "how hard is that to scout". A detailed read through the whole article shows that the process wasn't about finding the player, it was about deciding whether to choose the player - whether he was a good fit for our team ethic and our style of play, what Pochettino always calls the "philosophy".
To determine that, Mackenzie had to assess a wide variety of factors that were not revealed by which clubs had previously signed him, nor by his obvious playing statistics. That's the whole point of the article, not "how hard is that to scout".
The real "tell" with Alderweireld has always been the collective defensive records of every team he has played for, up to and including Spurs. He got on with his business in a non-flashy way and every team that included him saw a marked improvement in its goals-conceded record when he was on the pitch.
That made him an ideal signing for us: because no one ever seemed to give him personal credit for the improvement in the defensive collective, not until last season, when it started to add up and the press took notice.
And that's where the "scouting" lies. Not in finding out about Alderweireld before anyone else had heard of him, rather in recognising that his teams were defending better because of his presence and that his durability, reliability and character would fit into our squad.
EDIT: A few weeks ago, I posted an analysis on Alderweireld's thread, showing that we had conceded 5 goals in the first 10 matches of this season (before his injury), winning 7 of them and keeping 6 clean sheets.
Then we conceded 12 goals in the 10 matches he missed (including the one where he incurred his injury), winning 1 of them and keeping 2 clean sheets.
We have played another 9 matches since then, although he hasn't played all of them. During those 9 matches, we have conceded 5 goals, won 8 matches and kept 4 clean sheets.
Did you read any of the discussion in here? It's been explained in pretty clear terms. Scouting isn't as simple as saying "he looks a good player in this Southampton team, therefore he is guaranteed to be a good player in our Tottenham team".
Scouting at the highest level isn't really about finding players - it's rare that a player capable of playing at the highest level is completely off-radar (with notable exceptions like Kante). At the highest level, a target's ability is probably well known - at that point, the scouting is about minimising the risk of failure...now obviously the better a player looks at another club the lower that risk is likely to be, but every team is different in a myriad of ways that can affect a player's ability to perform at his best.
At it's most simple, this could be Madrid not showing an interest in Lloris - theoretically a better goalkeeper than Navas - because he's not good with his feet and so doesn't really suit their system, or Crystal Palace turning down the chance to sign a much-needed goalscorer like Defoe because their entire team is built around having a big lump of a front man.
At it's most complex, like choosing a centre back to form part of a defence, as in this topic's case, you're talking about everything from the basics -which side of the defence he prefers to play on, is he an organiser or does he need to be instructed on the pitch - to more subtle details; does he naturally stand off an attacker and wait for their move, or is he pro-active in trying to win the ball back, requiring his centre back partner to provide cover. That sort of thing.
Above all players, centre backs probably require the most scouting because their performances are most affected by the players around them. Attacking is, to an extent, about chaos - players using their ingenuity and intuition to find a way through (while obviously relying on a tactical base and method). Defending is about trying to control that chaos, and so a defence has to act as a unit; if one player's style of play is a mismatch with the rest of the back line, it won't work regardless of how talented he is.
Sorry for the essay. Long story short - Alderweireld is great, as he has been at his other clubs. But it doesn't always work like that, and scouting is a key part of the due dilingence of ensuring a transfer is a success, regardless of how 'obvious' it is that a player is good.