Offhand, I would try some sort of liquid sealant applied to both sides of the seams of the rainfly.  Or try seam tape - what have you got to lose? Do we know that this REI employee does in fact know what he/she is talking about?

Storage conditions vary.  Heat is especially hard on sealed seams.

One thought - how about using a sheet of seamless plastic sheeting as a rainfly?  A lot would depend on the configuration of your tent.  It would probably look odd, but might be quite effective.

I don't think that "non-REI" tents will be  different with respect to water sheding....

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