Not entirely true.
As an example:
OP takes out the bond today for 990k.
At prime (10.5%), over 240 months, the total interest is around: R 1,382,150.61
Total repayment amount over the period is around: R 2,388,566.61
If OP decides that they want to add
R500 per month, from today, the interest saved could be: R 230,149.97 over the period
R1000 per month, from today, the interest saved could be: R 386,820.71 over the period.
Furthermore, the total loan period could be reduced in both instances to 207 and 183 respectively. That's almost 3 and 5 years.
If OP doesn't add monthly, but does a single lump sum of R50 000, 1 year from today:
The interest saved over the period of the bond could be around: R 267,230.49
The period of the bond could be reduced by 32 months.
As much as these are indicative amounts, I'm not sure "it wont make a substantial difference" is valid.
Be it R230k or R267k, it is still a significant saving, as well as a reduction in the term of 30 months+
Yes, one can argue that the present value of future funds can come into effect, in which case don't pay the bond but invest the money elsewhere, but that would mean greater risk exposure.
A "guaranteed" reduction in the term and total repayment amount vs higher potential returns on a capital investment may seem like good odds. It should be carefully considered though.
My 2c.
(Disclaimer: This isn't financial advice, just a depiction of the possible impact of various decisions on a bond repayment amount and term. Other factors may influence this, including rate changes, increased service charges etc)