That "mechanic" is an idiot.
EPA-rated tanks have approximately 5 lbs relief, both for pressure and vacuum. Attwood (and possibly other) tanks that don't have a vent screw have a venting system in the cap. It works just as poorly as the Scepter-brand tanks that are used by Tohatsu, Honda, and other EPA-rated tanks that do have a vent screw. A Demand valve only ensures that fuel does not flow unless there is vacuum at the motor side of the hose, so, unless malfunctioning, they really aren't an issue, aside from possible flow restriction in high-flow motors, and the MFS6A/B/C certainly is not a high-flow motor, consuming a maximum of about 0.6 G/hr at WOT.
Kehin carbs used on Tohatsu motors are also used on Evinrude, Yamaha, Mercury, and other 4-strokes. They are repairable, and kits are available. They do not "blow out" if used with EPA tanks, but could possibly flood a bit if the primer is left connected, and the tank heats and swells up in the hot sun.
A bigger issue is that if using an EPA type tank, you must disconnect the fuel hose at the motor when not running, because the tank will pressurize up to 5 PSI when heated by the sun. That could cause a blow-out of the diaphragm in the fuel pump, or flooding of the carb, since the needle/seat is only designed for up to maybe 3 PSI, as delivered from the pump.
Actually, the earlier, non-epa primer hoses are preferable, as they are more supple, and last better overall. But they do allow some miniscule amount of vapors to leak into the atmosphere, hence the idiots at EPA deemed that the newer hoses must not allow vapor leakage to the atmosphere.
A no-start of the MFS4/5/6A/B/C should be diagnosed as always; determine whether the issue is fuel or spark, and make corrections as determined by that diagnosis. Pull the plug. If wet, either flooding or no spark; if dry, insufficient fuel.
