He's wrong. This is the jazzlab product, they just have a newer version that looks different. They may have sold their old versions to a warehouse to liquidate them. This is the official updated product, though I think you'd be happy with the one you showed.
Hey, I'm starting marching band this year as well at 14, but I play tenor saxophone. I actually have this neck strap that I have had about a year which is built with adjustable plastic pieces that puts the weight on your shoulders instead of your neck. It's called a saxophone holder pro. It's been a help for me and it actually in my opinion makes it easier to march and play while standing. Here's a link to one on Amazon. https://www.amazon.com/Jazzlab-SAXHOLDER-Saxophone-Harness/dp/B005J3EZZY/ref=mp_s_a_1_4?dchild=1&keywords=saxophone+holder+pro&qid=1622389691&sr=8-4 They can be a little pricey, but if you plan on continuing on in marching it's worth the investment. I will say it also makes playing in concert band a lot nicer because after an hour long concert you don't feel like your neck just got snapped by SCP-173.
Depends on the context - are you moving around a lot while you play (e.g. on the street, in a marching band) or are you mostly just standing/sitting still while playing?
If you don't move much when you play, the Jazzlab SaxHolder is a popular one - it's super lightweight and easy to use but keeps the weight off of your neck as effectively as a regular strappy harness (I actually use it primarily for my bari). Only issue is that it's not looped around your body and has an open hook, so you can't exactly run around and dance with it on.
If you're out and about and grooving, you can always get a basic padded Neotech neckstrap. The biggest downside is that the padded part collects sweat and gets gross so if you're out playing in the hot sun a lot, you're gonna have to hand wash and air dry it regularly. You can also do a standard harness like this, but they're a little bit difficult to put on because of all of the straps and buckles (and tbh they do look dorky if you're not playing a bari...).
Also, depending on your reasons for avoiding leather, you can always look for a secondhand leather neckstrap.
I understand that this is a bit late but I've found myself lucky enough to march with two other bari saxes, especially as my main instrument is the oboe ( I picked up bari in January 2021).
Saxholder- get one that locks all three of us march with one and it helps to distribute the weight across shoulders and your stomach/chest area. The only downsides to a saxholder is sitting down or if you march bari diagonally, since my band marches straight in front of us the saxholder is a life saver but to the side its a bit iffy, however I still use mine when I sit for both bassoon and bari so its plausible. The only other problem i find myself dealing with because of the saxholder is slides make it get slightly dislodged, but other than that I don't have many complaints.
I'd also recommend a wrist compression sleeve for your right hand and a compression glove, using both work absolute wonders for me (I have tendonitis in both wrists) and even helps one of our other bari players. Also investing in a good right hand carpal tunnel brace if your wrist starts to hurt and wearing that while not practicing work wonders.
Stretch too- I don't know how well emphasized this is in your band but finding good wrist, arm and back stretches work wonders. I have a few particular ones I like for my back and they both help loosen it but also prepare me for slides too.
Saxholder link: https://www.amazon.com/Jazzlab-SAXHOLDER-Saxophone-Harness/dp/B005J3EZZY
just $20 more at $50 BTW don't ever buy something on Amazon with no reviews
Can't you play electric bass?
This is the new hotness in the realm of harnesses. Here's one of my favorite sax players using it