I have recorded a modern Thinline reissue with humbuckers, and that did sound very muddy. But I'm told that the original Wide Range humbuckers are completely different and much better, never tried them though.
Completely different. The genuine article are the opposite of muddy, and actually remind me a little bit of a juiced-up Fender single-coil.
Here's the total story of why you heard what you heard and what's up with the pickups on those.
Gibson-style humbukers of Seth Lover's 1957 design are constructed like this: Two coils of opposite winding direction are wound on separate bobbins, and connected in series. One coil is wound around steel "slugs" and the other coil is wound around adjustable screw polepieces. None of the polepieces--slugs nor screws--are permanent magnets. They're just plain rods/screws of steel. Underneath both coils, under a spacer, sits a permanent bar magnet of AlNiCo II, V, IV, or ceramic ferrite. One side of the bar is "north" and the other side is "south." The north side contacts one row of polepieces (say, the slugs) and the south side contacts the other, meaning a single magnet temporarily charges both rows, but in opposite magnetic polarity.
Fender Wide-Range Humbuckers (also Seth Lover's design, incidentally! He defected from G to F) are an entirely different animal. Two coils are wound on slightly-larger bobbins, generally with a higher number of turns. Each coil is wound around six permanent magnet polepieces-- three adjustable threaded rods with screw-heads, and three non-adjustable slugs. One coil has the adjustable poles on the bass strings, the other has them on the treble strings. Because AlNiCo alloys and ceramic ferrite are too brittle to be threaded reliably, the polepieces were made of an alloy called CuNiFe (copper, nickel and iron). This couldn't hold quite the same level of flux as AlNiCo, but was an acceptable permanent magnet. A Fender WRHB is, in practice, very much like two Strat pickups with CuNiFe magnets connected in series with reverse winding and reverse polarity. The output is higher than a vintage PAF.
There are some other minor construction differences as well, but that's the gist.
In the modern era, CuNiFe magnets are hard to come by, for whatever reason... It has few industrial advantages anymore, and the supply chains just don't really exist. So for the thinline reissues (which were budget import guitars anyway), modern Fender just took some VERY cheap ceramic-magnet Gibson style humbuckers, put them in the large WRHB chrome covers, and filled the enormous void under the cover with tons of wax potting. They dolled them up so that the screws appeared in the right place. They will pass signal, but what they are is very cheap ceramic PAF-style buckers with bar magnets, short coils, lower output, lower turns, and just generally no relation at all to a true WRHB.
Jason Lollar, Curtis Novak, and a guy who goes by Telenator make proper WRHB re-creations now, which did not exist until fairly recently. Telenator has actually found a source for CuNiFe magnets, but they're very expensive. Lollar and Novak have sourced threaded AlNiCo magnets, which are possible to make now due to advances in tooling. Any of the three are a MASSIVE upgrade to the "fake WRHB" used in the imported Fenders, and actually make them into something that begins to resemble a real thinline.