That would imply that the gameplay is similar to Crysis and the mech is just a playable vehicle like the battle tank is in Crysis.
And all players start as infantry with gun?

That is not a nice thought...that would mean we would have Crysis and just a bunch of new vehicles and maps.
Besides that if you are infantry or BA would be the same. A hit by PPC, LL or ML would instantly destroy both unit types, tanks probably would sustain 2 PPC hits maximum. You would stand no chance at all against a heavy or even one medium mech fitted with PPC or LL.
Someone has no clue how BA and Vehicles really work in BT. For reference, the Elemental has 10 armor + 1 more for the trooper, which means that one elemental will take 3 Medium laser hits to kill or 2 PPC/LL hits assuming you are not using the Clan ERPPC, Clan HLL, or IS HPPC.
As for vehicles, they can mount an unlimited amount of armor, and it is all concentrated in 4 or 5 locations so depending on the design they can take more firepower to kill than a 'Mech. Their weakness is that they suffer badly from critical hits through armor and hits to their drive systems, so you may not need to kill them to take them out of battle. They also don't like heat based weapons like Flamers and Inferno missiles which can disable or kill them in a few shots.
As for MWLL, the BA is used to replace the pilot so that you don't have to fight on the ground as infantry because they really do get instantly killed by anything that hits them. You have to buy equipment if you want something other than the basic BA, although you do spawn with money so you can buy a basic tank or 'Mech at the start of the game if you want to. The only reason you would fight as BA is if you chose to not buy something or ejected from your 'Mech or vehicle.