I don't expect that the developer did anything that would require players experienced with SMW to change their memorized routing. (If I were them, I'd be using 100%-playthrough TAS .smv files from the unmodified SMW as conformance tests, to ensure none of my changes "desynced" existing strategies for the game.)
What might be different, on the other hand, are subtle things about un-loading. I bet an .smv of the SMW credits warp wouldn't work on this patched ROM. (Not least because, IIRC, this patched ROM is based on an older set of SMW patches by the same author, that — among other features — has a sprite-limit removal patch, that improves the game's OAM memory-management algorithm.)
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But also, IIRC, the enemy/platform spawning (really, the loading seam) in SMW occurs far-enough outside of the viewport normally, that it might still be outside of (or just at the edge of) the expanded 16:9 viewport.
Recall that in SMW, you can explicitly scroll the viewport in the game (by pressing L/R). The game engine was necessarily designed around the assumption that, at any time you're in a free-camera scene (most of the time when you're not in an autoscroll area or small room), your viewport has an arbitrary horizontal scroll offset within some defined range. For gameplay to be deterministic/testable under such an assumption, the loading seam must be programmed to occur outside of that manually-scrollable region.
I'm guessing that the basic "insight" of this widescreen patch, is that in most areas of the game, the screen can be made exactly as wide as the entirety of the coverage-area of the camera's player-controlled scroll region, with no effects on gameplay; because the game was already designed to work "the same" within any sub-region of that area, and "all of it" happens to be such a sub-region. Presumably, this patch makes the screen exactly that wide, and then removes your ability to scroll the camera any further using L/R. (Without that last bit, you'd be very likely able to see pop-in by scrolling the screen even slightly.)
This would also mean that any area that was designed for a fixed or restricted camera (e.g. autoscroll areas; boss rooms; etc), may have been optimized by the original level designers under that assumption, and so "packed" with stuff that might load at the wrong time, or in the wrong order, if the loading seam were expanded (as it likely would have to be to make these areas look good.) Not having looked yet at the developer's progress reports for this patch on Twitter, I would pre-register a guess that fixing these types of rooms is where the majority of the work in making this patch went (beyond just adding more border tiles to small rooms.)