Relative to everybody.
Megapack is a product they've been perfecting for many years and it's already at least a v3 product.
Not only they are not using Panasonic cells in Megapack (those go exclusively to their higher-end cars), if you think all it takes to create a competing product is to have money to buy battery cells then you exemplify "a little knowledge is a dangerous thing".
Megapack is a product with lots of sophisticated electronics that takes serious engineering to develop, it's at least v3 product with years of improvements. You need high-powered voltage conversion, battery management hardware and software so that it doesn't explode etc.
Tesla is a clear leader in the field: every major energy project announced, from Australia to California, uses Tesla megapack. I don't hear much about any of the competition.
Tesla is leading in scale and expanding rapidly (just finished ramping up first U.S. factory and building another factory in China).
Even for Tesla to build another 40 GWh factory which is mostly copy & paste of their existing megapack factor, it's a year long process.
If you start with battery cells, you're looking into a multi-year design & testing & initial production for the first megapack-like product and then at least another year to scale your first factory to 40 GWh yearly production.
By which time Tesla will have few more factories and v5 of Megapack.