Ben, your surgeon is describing standard practice for a second session in an average donor, not a step backwards, and the reason it sounds odd is that the marketing has turned a harvesting choice into a badge of modernity.
The direct answer first. FUE and FUT produce the same grafts, the natural 1 to 4 hair follicular units, and once placed they grow the same way; what differs is how the donor is harvested and what it leaves behind. FUE takes units one at a time with a 0.7 to 1.2 mm punch, spread across the whole safe zone at a density commonly kept to around 10 to 15 excisions per cm2 so the thinning is invisible. FUT removes a strip about 1 to 1.5 cm wide from the densest part of that zone, the area is closed with sutures or staples, and the strip is dissected under magnification. The consequence your surgeon is weighing is that each FUE session lowers the density of the entire donor band a little, whereas a strip removes a narrow slice and leaves the density either side of it untouched. In a donor that started at roughly 65 to 85 follicular units per cm2, which is the typical safe-zone range, and has already given up 2,900 units to FUE, a second wide FUE harvest is where the sides start to look patchy under a short clip. The strip avoids that, and it also leaves un-punched scalp above and below the line that FUE can harvest years later if a third session is ever wanted. That is the "in reserve" remark.
What it costs you is precise and you have named it. A linear scar, typically fine in good laxity and good closure, but permanent, and hidden by hair rather than by absence of hair. The NHS phrasing is that it is not visible unless the hair is very short, and a grade 2 is usually on the safe side of that line, but "usually" is doing work there: scar width depends on scalp laxity, on tension at closure, and on the individual, and it is a fair question to put to him directly, along with a look at healed donor photographs from his own strip patients at the length you wear. Recovery differs too. Expect tightness across the back for the first week or two and sutures or staples out at about 10 to 14 days, against the wider clipped area and hundreds of small crusts you remember from FUE. The comparison at greater length is in FUE versus FUT, and how the finite donor gets spent across a lifetime, which is the whole logic of his plan, is in the donor area and overharvesting.
For calibration on "old": FUE has been the majority technique in ISHRS practice surveys for some years, but strip has not disappeared, and surgeons who keep both in their hands use strip specifically for large second sessions, tight donors and patients who want to keep length at the back. Crown work eats grafts, and 1,800 for a crown in a Norwood 3 that is progressing is a sensible, conservative number rather than an ambitious one. What I cannot judge from a forum post is your laxity, your density under magnification, or how your first donor healed, and those three things are exactly what his recommendation rests on. He has examined you twice. I would ask him the width question, ask to see his healed strip donors, and then let the answer be about your scalp rather than about which decade a method belongs to.