True ? cableTV and cableINTERNET vs only internet is faster?

i only have comcast high speed internet

someone knocked on my door, he was from comcast, and saying they are doing some cable upgrades, and notifies my, asks if my internet has some problems, i replied no, not much. then he advertises that i can get basic cable tv for 5 dollers more, and it will boost your internet speed, "because the modem has to separate the two channels", and only having internet makes your modem slower, having cable tv and internet puts less strain on the modem, makes your internet faster.

is this true or just a way to convince you to get their tv service?

He's full of sh!t.
Hook a tv up to the cable line and see if you can get basic channels. If you dont then they have a filter on your line to cut out those channels/frequencies when proves him wrong about "seperating the two channels"
If you do get channels then he is full of it because you signals wont change if you did officially get cable.

The modem works just like a tv tuner or radio tuner. It only looks at certain frequencies and ignores the rest.

so its a way to convince you to get cable

and why do you want them to knock your door?