SW Vacations is an authorized Disney package seller. Dining plans are listed as an add-on for your package. All documents are E-docs, so other than ME tags you don't get any Mickey Mail, vouchers, etc.
If your question is about
FREE dining, then changes are handled very differently from the way Disney CM's handle it. SWV does offer the FD programs when they come out for the general public, but they are a different inventory and package code. Would entail a cancel/rebook of your land portion which could result in penalties if you had to change your reservation to FD or another promo discount after you originally book your package. All changes would be thru SWV, you could not call Disney to add
DDP or make any changes.
If you had a booking directly with Disney then the CMs just go in and change your res if FD inventory is available, no cancel/rebook, no penalties other than a small change fee if you are doing this after your final payment date.
So the answer is, yes SWV is a reliable place to book IF you are not going to change your reservation in any way, including changing for promotions. But booking directly with Disney gives you much better flexability for any changes.