If the tail supply fee is redacted from the transaction fee, then there's no actual tail emission, as described by o_e_l_e_o. The supply remains 21 million.
It is all about proportions. You can have two systems:
1) with fixed supply, where everyone will lose some satoshis in explicit way, and they will be taken by miners, because of tail emission
2) with infinite supply, where everyone will lose some satoshis in obscured way, because miners will be always rewarded by new coins, because of tail emission
Maybe you can't see that, so I will try to make some more extreme example, you can adjust numbers to reach some real-life scenarios:
Imagine there are 21 million coins, distributed to many different users, and the block reward is zero. Then, imagine that 21 million coins are produced, because of tail emission. Then, you can have two systems:
1) with fixed supply, where everyone will lose half coins in explicit way, and they will be taken by miners, because of tail emission: 10.5 million coins will remain in users' hands, 10.5 million coins will be taken by miners, 21 million coin limit is untouched
2) with infinite supply, users will have exactly the same amounts, but they will be worth 50% less than before, because miners will be always rewarded by new coins, because of tail emission
And then, imagine more real-life numbers. What result you will reach? You can do some simulations, then you will see that taking single satoshis from all accounts, and building new block rewards from that, is an equivalent to tail emission. It is the same, if you know all addresses and all amounts, then you can exactly calculate, what percentage of the total supply each user has, and then you can calculate, how many coins each user should own, to reach an equivalent situation, in a fixed supply coin.
It is simple. The total supply does not matter, all that matters is that is a fixed number. You can create a coin, where all amounts will be numbers from [0;1] range, then you can add fractions, and then you will explicitly see, that if we have 21 million coins, and if you burn 10.5 million coins, it is the same situation as doubling all balances on all accounts. And by contrast, if we have 21 million coins, and if you produce additional 21 million coins, it is exactly the same, as halving all balances on all accounts.
Then you should understand, that adding for example 0.01 BTC to the system as a tail emission, is effectively the same situation, as taking single satoshis from all accounts, and giving that to the miners. The difference is simple: it is done in a more obscure way.